This may sound weird, but I find Protoss the easiest to expand with.
Protoss have a couple of nice things that help expanding:
- Cheap static defense
- Warpgates to reinforce anywhere they have power
- Pylons provide power across building walls and cliffs
While I definitely feel like I must expand when I play Zerg, I feel most comfortable doing it as Protoss. Usually this means I place buildings at a near choke point (usually Warpgates, but I'll use the Forge if I don't have one and I'm walling my Natural). I then augment this with a Photon Cannon behind the wall to deal with light harassment. Finally I place power so it overlaps my wall to outside, that way I can easily reinforce. Now you may ask: what happens if you go Robo? and the answer is, I usually either break the wall in late game, or use Warp Prisms to move Immortals across (Colossus can walk without help), but more often than not, I just don't build Immortals; they aren't a part of my normal builds (I don't like the 1-2 Immortal timing pushes against anything but 1 base Roach).
That said, there are a number of ways in which expanding is very nice and easy for Terran.
- Fly in Command Centers (which can then be made in to Planetary Fortress)
- Cheap walls with Bunkers (can salvage)
- Supply Depots are removable walls
- Siege Tanks
The largest problem I have with these expansions is light air harassment. Either I have to over invest in Vikings to shut it down, or I have to over invest in Turrets. Ideally I'd like to have a nice Marine/Medivac force to deal with this, but in reality moving large groups of Marines through Supply Depot walls takes a lot of time, and Medivacs don't unload instantaneously. Not that I can't do it, I just feel less safe doing so
As Zerg I don't mind moving Spine Crawlers to my newest expo (which usually means 12 second build time) or even between me and my opponent, but I largely feel like I need to have map control before I can safely expand. Since most Terran/Protoss are willing to give it to me that's not a major problem, but my expansions feel inherently unsafe. So in the case of Zerg I would say: Only expand when you have map control.
This is very much the traditional BW philosophy and it has serve many players well over the years. I would guess (and the meta game evolves and Blizzard balances a few things) that this is going to become the only strategy. While Terran and Protoss players will probably still build structures defensively, the cost/benefit of having to wall is much higher than the cost/benefit of gaining map control. Then again, that's a prediction and is inherently flawed.