Wall-in: benefit or vulnerability?

Solution 1:

He's referring to the fact that you've placed a few supply depots and a rax right at your main base entrance. It's a vulnerability because it's 16 supply and a production building that is much easier to snipe than the rest of your supply and production buildings. In essence, you've given your opponent what Day[9] considers to be some free buildings to destroy.

As to whether or not its a liability, it depends on whether or not you extend your defensive perimeter to include your expansion bases.

Edit: Example of this weakness in action (playing the video will jump you to the relevant moment in the game):

Solution 2:

It also creates a choke in your own base, so for example if your army is in the main base while your natural expo gets attacked, it will be more difficult for you to engage the opponent, because your wall-in forces your army into a small choke.

[update]

Note: In team games (2v2 / 3v3 / 4v4), it's far more compelling to wall in because you risk facing much stronger rush attacks. The bigger the rush, the more important it is to keep them at bay while you prepare a defense. As Terran, I sometimes place unfinished buildings at the front as an emergency, or as Protoss, you sometimes need multiple zealots ready to do the hold-position gate.

[update]

A wall-in can often provide void rays an easy full-charge.