No doubt a higher level player could say more, but there it is.
Time to learn about the manly race.
Probe- as weak as they are, you wouldn't think they would have much combat power (compared to say the SCV.) However, if you think Huk's probe is irritating, you've seen nothing until you've seen the Bisu probe. This is largely due to the perfect speed between probes and most of the tier 1 units. As long as the probe keeps moving, they cannot be caught. Not too slow that they would be immediately killed and denied scouting. Not too fast so that it is no challenge to keep them alive.
If you shift-click waypoints, the probe will slow them down so zerglings, etc can catch up. Therefore, the probe must be constantly on the move and with the right control can even kill off workers. But they are often just a couple seconds away from dying if left alone.
It is also not uncommon to see the scouting probe fight along a couple early pressure zealots to decrease the zealot hits by one to kill.
Core
Zealot- The first unit that gets saddled with the 1a2a3a label. And certainly most beginner Protoss' will stumble upon some version of the 2 gate zealot rush. However, I think Plexa writes it best that zealots are made interesting by their relation to other units. Unit Interactions These are tough buggers and just 4 zealots dropped into a mineral line will wreak significant damage.
They are very good at clumping and so it is all about spreading them out with conga lines and magic boxes to negate the massive amounts of splash damage that will finish them off. And you need to spread them out to turn Terran's splash against them: spider mines and Overkill tanks. And then there are zealot bombs
Dragoon- The second unit that I think is unfairly labelled 1a2a3a unit. They are also the #1 examples used when people want to complain about BW units. Certainly they do both, but that is short-changing them.
The great thing with the dragoon is its hold-position micro. The early game is often composed of short intense micro wars as the dragoons attack-retreats back across the map. Get four or more of them and suddenly the retreat will about turn and pick off a tank or two. Then you combine the dragoon-vulture micro wars, particularly when dragoons are forced to pick off spider mines without the aid of vultures. It is the fine line between awesome skill and having it all blow up in your face. Preciseness and skilled control is what makes dragoons great.
The two shall become one.
Together zealots and dragoons form the core of the Protoss army. vP and vT, dragoons will typically be massed first with zealots added after. vZ, Zealots will push out onto the map after surviving the early game to establish to threaten any vulnerable bases. Zealots will bear the brunt of zergling and tank attacks, while dragoons will clear out vultures and lurkers. Both form a sort of symbiosis protecting each other from their respective weaknesses.
But if we just had those 2 units we could truly call Protoss 1a2a3a and leave it at that. And a boring race it would be indeed.
Support
High Templar- I absolutely love the ht. So much raw power placed in my hands for me to personally control. It is hard to get off a lot of storms, but each storm will shred the enemy to pieces. As a result, the ht is used to defend expansions with nought but cannons and a handful of units. They are brought up for the main attack, to lay down their destruction. The core army of Zerg will melt to these storms and most of the Terran army will be shredded. This in turn forces hydra storm dodging.
They are consistently used to level worker lines. One storm will destroy half a worker line. Two will destroy them all. Of course the enemy knows this and will scramble to save the workers and so it becomes a head-game of where the workers will run and where to best drop the ht to storm them all.
Storm drops are perhaps my favourite thing about Protoss and if you agree or you are curious, you should watch today's TLS with dRaW.
Archon- Tanks a lot of damage and can murder a lot of zerglings with splash if it doesn't get surrounded. One of the interesting interactions is Muta vs Archon. The Archon is pretty fast and if it can ever catch the muta they'll feel the pain. Especially if they already ate a partial storm. But the muta are faster still, if they keep moving. Beware to the mutas that pause and there is an archon or two nearby.
Dark Templar- This is the unit that gives Artosis nightmares. This is the unit that gave Bisu his reputation as multi-tasker extraordinaire. To control a corsair fleet, independent of multiple dark templars harassing across the maps while macroing... my poor multi-tasking cannot keep up.
DT's are truly fearsome as they can be upgraded to one shot workers- particularly drones. So while the corsairs clear the skies of Overlords, the DTs slip in and assassinate workers. It takes an aware opponent to catch them. But they are expensive to lose and one would not wish to lose them without causing some damage.
Dark Archon- Rarely made due to cost and efficacy, but I'm fine with this. Maelstrom is good on occasion to stun bio units and then storm them. Anymore prevalent and it would be very irritating to play against.
All of this and we are just on Gateways. Why ever would you go Robo with only one slow attack unit in it?
Robotics Facility
Observer- Pretty straightforward what obs do. Very important for map awareness, but long lived probes and corsairs means you won't die to something completely un-expected despite needing an observatory to get obs.
Shuttle/Reaver- One might as well say the Reaver lives in the shuttle. Without one or the other, neither would be near so epic. It is one thing to have a shuttle. It is another thing to have something worth putting in it for harass (dragoons aren't it.) There are just so many options to harass with Zealots, HT, DT, Reavers. The harass becomes especially potent once speed is upgraded.
But Reaver harass is a special sort of harass. You often need to be constantly moving shuttle to keep up speed. With burst shot and splash damage, the Reaver fires off one shot and can be immediately picked up to fly away. That one shot if aimed correctly can explode a great stack of workers. Or perhaps pick off stray units, knock out tank clusters, etc. It is the fencing sword of the Protoss arsenal. Dancing in and out, looking for holes to exploit. Dropping zealots to force the tanks to fire, dropping the reaver behind, picking it up and racing out again before the tanks can fire.
Honestly it is one of those things that must be seen. It cannot be properly described.
Reavers are again a unit that can sit behind a cluster of cannons and face the oncoming Zerg swarm. They can be used to harass the enemy mineral lines or tech buildings or reinforcing units. They can pick off reinforcement lines. They can push up with the main army to bulldog down the front door. They can join the main army. However, their micro-intensiveness prevents them from snowballing out of control. Corsair-Shuttle/Reaver is among the harder Protoss builds.
Air
Corsair- stack them with a trapped worker and clear the skies of Overlords and fight back the Muta. I should probably do an entire blog on the scourge, but the speed difference between scourge and corsairs and shuttles is just perfect to create such tense air maneuvers.
Carrier- When (or before) mech Terran gets too tough, there is always the tech switch to Carriers. What could be a boring a-move unit is saved by a very cool micro trick (described best by NoNy Carrier Micro) Their counter is typically the ground-based goliath. This Air to Ground battle means positioning is everything. Cliff abuse and endless attack-retreat micro is the name of the game.
Arbiter- Mech play (mine fields, cannon fodder and harass units, anti-air and the all-mighty Siege Tank) could easily have become an unbeatable ground force. However in the mid to late game, the Protoss' best mobility weapon can be brought online.
Cloaking fields- Arbiters are fast and cloaking fields allow your units to close the distance to get into range of the siege tanks. At a certain point range attack units reach a critical mass and it becomes difficult for melee (zealots) to be effective. Cloaking fields help melee over-come this issue.
Stasis- Too prevalent and this too would be irritating to play against. However, they are late game and cost a lot of energy... and there are patrolling Science Vessels with EMP. Stasis can best freeze out clumped units (thus forcing spread on tanks and the like.)
Recall- Late game recalls gives Protoss tremendous mobility in the lategame. Terran gets ever so strong that it becomes hard to fight them directly when they are set-up. Speedy arbiters move around the map and recall in multiple locations, pulling the Terran army in multiple directions. This forces the Terran into engagements where they are not necessarily entrenched.
Scouts- The humiliation unit. Build it to diss your opponent or to do the Stove http://www.teamliquid.net/forum/viewmessage.php?topic_id=94911 . Although if Guardians appear, they can be useful. Also, I understand that on big money maps allying with Terran and going Tank, Cannon, Scout is pretty powerful. They also can be stacked with a trapped worker. With their burst damage and speed upgrade, they are pretty manoeuvrable.
Big Picture/Summary
What I like about Protoss is there great mobility and the variety of units that can be used not only to attack, but also to defend, and to harass. Blizzard may have first described them as the expensive, hard to replace army. And that may be true vZ, but vT, Protoss is not the thin-red line, but the enveloping flanks of the Zulu.
The aggressor-defender may shift during the game. (vZ Protoss fast expand, puts Protoss on the backfoot, but soon after zealots can be ranging the map.)
Protoss is mobile. Protoss forms concaves. Protoss surrounds. Protoss forms concaves. Protoss harasses with Corsairs, zealots, reavers, high templar, and dark templar. Protoss attacks at different different angles and tries to find weak spots with Arbiters or Carriers. For all these reason more I love Protoss. The race of Dragons, the race of the Man Toss and the Manlot. The race of the Revolutionist, the Commander, the Dreamer, and the Almighty.
Protoss Fighting!