[M] (2) Nightmare Hollow

Protoss can get three bases rather swiftly, but the bases they chose provide vulnerabilities. If they decide to take the pocket natural first they will become vulnerable when expanding to the open natural. Start taking down the rocks, and when they do try to expand they will have to spread their army thin to defend. Expanding to the open natural first is harder due to the open nature of the expansion and abusable cliffs, something you can use to your advantage to punish their greed.

Air harass is strong on this map due to the layout of the bases (3 mineral lines close together and accessible by air) and the long rush distances, so phoenix openings may be more viable than the average map. For harass lategame I saw 4KExO place a Phase Prism on the Xel'Naga like hallway, sending 3 zealots to the raised, gold, and natural expansions.

Zerg can expand swiftly. Be ware, any fast expanding on your part will often be met with a double-expand from them. Pressure to punish them and keep then from properly droning to 3 full base, or surprise them with a powerful 1-base push. Parts of the middle are wide open, so keep your back to a cliff or the acid pools in the center, and beware of counterattacks. Also, there are plenty of lovely high-ground patches for Zergs to place their overlords. Don't build your tech at your ramp if you don't want it to get scouted, or make sure to clear the overlord off the nearby high ground.

Terran have many options for early-game pressure. There is an almost lost-temple like high ground next to the open natural and I'm sure terrans will have plenty of fun with that. Or, they can even forgo Medivacs and begin their assault on the open natural by placing tanks on the raised expansion. Due to the layout of the bases, banshee harass is strong throughout every stage of the game. Siege tanks can be used to lockdown defense, but late-game favors mobile army compositions due to the size of the map and the strength of two-pronged hit-and-run harass. Locking down the defense with a several extra Planetary Fortresses can lock down their side of the map and free up the terran army to get aggressive. Building a stupid amount of barracks can help replenish harassing armies with a zerg-like efficiency -- after all, there are a lot of bases on this map, you'll probably be able to afford it.

All
The in-base Xel'Naga plays a critical role in adding stability to the map. Most maps with back door rocks (Jungle Basin, Blistering Sands) are trash, and create an "oh crap" moment where a poorer player can win through silly shenanigans. Not only is the defense of the back door easier here (especially if you don't take the pocket natural), but the Xel'Naga give you forewarning and army composition. Once the rocks are down, the Xel'Naga gives you forewarning if the attacker doesn't take the long way around to harass. It also reduces the air threat like muta harass, forcing the mutas to take longer paths around to hit the base lest the tower scout them. So don't forget to use it! (Also, Protoss, you can place a pylon next to the destructible rocks and warp in to their pocket natural, but the Xel'Naga tower will scout it. If you know they don't have the tower, though...)

The raised expansion has a gas near the cliff facing the enemy. This vulnerability can be exploited by all three races, sniping off the extractor and limiting the player's gas income. Unlike Tal'Darim alter, however, this is a 3rd/4th expansion, so the effect is less extreme while being easier to execute.

And one final note on the gold, watch for ninja expands there. They're not on the main path or rocked, so it might just happen. But, if you or your opponent does take the gold, defenses will be stretched thin: it's a LONG walk distance to defend the gold and the back of your main base.

Aaand an image illustrating the choice for 3-basing based on what natural expansion you took.

Since version 0.7:
-The unpathable high ground near the ramp to the main has been moved back, forcing the Zerg player to reveal his overlord to scout structures used to wall off.
-Serious scouting concerns for Zerg arose due to the amount of build space in the main and pocket expansions, and because the inbase Xel'naga limits the angles a Zerg can scout from. To resolve this, unpathable high ground has been added on the outside of the pocket expansion, allowing A Zerg to float his overlord there similar to The Shattered Temple.
- The ramp to the raised expansion outside of the choke has been been shifted to point in, allowing Terran to defend a bit easier due to the nigh-impossibility of slow-pushing on this map. (Tal'Darime Altar style pushing is still viable, though.)
- The center has been made just a bit more choke-y. Originally zerg could get extremely easy surrounds. The new one makes pushes somewhat less risky.
- A center expansion added, allowing Terran and Protoss to expand toward their opponent. (Terrans really were in a lot of trouble on this map in version .7!).
- Extra space around the gold was removed, and the gold base move a little bit closer to the opponent. This makes it a bit harder to defend fpr terrans (less of a choke), and creates a more tight and efficient map.
-The pocket base has been moved closer toward the center, removing wasted space and making a more tight and efficient map.
-The water areas between the main base and the center have been made larger. Originally, protoss could wrap around and crush any force threatening their back rocks, trapping the other player. With the wrap-around travel time increased, the attacker has time to retreat if he scouts it and is quick.
-To that end, a second retreat path has been added from the base outside the pocket expansion. This allows faster access to the enemies gold, as well.

And half a dozen more small tweaks...