EPTIf I had to rank the stats of a unit in order of importance, I'd put it in this order
Range
mobility (speed/air/cliffwalk/blink/charge/stim etc)
dps/splash
survivability
Range is at the top due to the simple mechanics of the game making each ranged unit added to a group add more power to the group than the previous, up to a soft-cap related to the range of the unit and how open the space you're fighting is. This makes it difficult for blizzard to balance units throughout the course of the game though, as to make a melee unit worth using vs a large number of ranged units, it needs to be strong enough that if the ranged units DON'T have that critical number, then it just walks right over them, as shown right now with the zergling early game dominance and then fall-off towards mid-late game. This is generally attempted to be resolved with upgrades and other extreme synergies, such as were seen in broodwar with the much more powerful adrenal gland upgrade and dark swarm.
Mobility comes in a healthy second place. One of the only things the mutalisk has going for it is it's extreme mobility, but that makes it one of the most used and feared air units in the game, allowing it to strike a mineral line and then escape within seconds. On the other extreme we have the thor and the siege tank, who are statistically two of the most powerful units in the game, but their slow speed and siege mode transition time bring them into parity with the rest of the units. Where mobility is really hurting right now though is with the hydralisk and the ultralisk. These units by their numbers are actually frightening, but their low mobility makes them situational at best, and a game loser to make them at worst. The broodlord also has a slow speed, but this is made-up-for by it being both flying and having an extreme range. Protoss is very much in the middle of the road on mobility with most of it's units, not quite mobile enough to take full advantage of it in terms of harass except possibly with phoenixes, but also never at a disadvantage due to low mobility either. Once you hit late game however mobility takes an unfortunate back seat, as units trade away a lot of their staying power in an army for that mobility.
dps and splash are very similar, but markedly different in their effectiveness as the game goes on and armies get larger. early on, with small unit numbers splash is relatively useless. Splash however increases in power as you get more units on each side of an engagement, up until you cap out your targets hit with the splash damage. additionally attack speed and missile speed play a large role here, as overkill can mean a lot of wasted dps. There is a good demonstration video(shown below) of just how much the tiny missile delay of a hydralisk can affect the outcome of a fight. This is a less extreme scenario though, if you combine both a slow attack speed and a missile speed this gets even worse, as seen with roaches, where a theoretical example of 8 roaches attack a marauder, doing 120 damage, leaving the marauder with 5 hp, unless the player micros and has only 1 roach attack that marauder for the next volley, 115 damage will be lost as all 8 roaches attack that same marauder on their next attack. high mobility allows for good micro to minamize the negative effects of low attack speed, muta micro is good example of this in BW, but we don't see much of it now.
Survivability is last on the list, as while it can definitely be important, it is more often that a unit will live or die on it's other merits instead of how much actual damage it can actually soak up. Ultralisk thor immortal, the 3 units in the game specializing in survivability. The thor is mainly used for it's high range splash, or it's high dps medium ranged ground attack though. the immortal sees sparing use right now, used primarily for fighting off stalkers early game, or for breaking tank lines late game TvP. The ultralisk however falls flat on both range and mobility, leaving it as simply high damage and high survivability, but that, as shown by how hated they are by zergs, is not enough.
Something that has baffled me is that all units that deal bonus damage to armoured are in fact armoured themselves. This has created an oddity with the remaining units. The roach for example can be said to counter any unit that doesn't do bonus damage to armoured, even countering stalkers to a degree. The same is nearly true for light, exempting thor anti-air and banelings. As it stands there isn't a lot of actual countering done by the bonus damage mechanic, except for the fact that everything dies faster, which in the end simply favors the higher ranged units. Though getting down to it, I don't know if we want an influx of bonus to armoured light units, or vice versa, as it would then create a much more hard-counter system than what we currently have, which is fairly muddy.
Higher tech units being lower food?
There seems to be an aversion to making a unit that you get later in the tech tree being something you mass more, which i think is a flaw that is limiting the game's design. For protoss I can see building up towards some grand fleet or what-have-you, but for zerg, I don't see why there can't be some hive-tech unit that isn't a massive death machine costing 4+ food.
This rambled and didn't quite get to a few of the points I wanted to make. A lot of musings and not a lot of conclusions, sorry about that.
