PS. how is the infected AI better? Do you mean the special AI or the common AI? Because the commons are still just as dumb as ever before.
Special AI, common AI doesn't really matter too much since they don't really do anything except attack you, a lot of the Common Infected doing anything relies on the director. The Special AI is vastly better though, they try to group attack as much as possible (more and more as the difficulty increases) and they try to combo together when they do so. There's some individual AI improvements like tanks will avoid fire (no longer speeds them up in Campaign) and don't hit downed Survivors anymore and Boomers will try to get you to hit them after they puke more etc, but a lot of is team work AI improvements.
A good example, this happened the first time I played it, was we decided to shoot a witch in the hedge maze for the hell of it because we were having such an easy time on Advanced. Exact moment we shoot her, hunter pounces one of our guys, Smoker smokes another guy, witch downs me, Boomer pukes on the 3 of us, then the Spitter spit on all of us so the last guy couldn't even do anything when we were downed. Even in the most luckiest of circumstances this would never happen in L4D1, they sat and waited very quietly around the witch area just waiting for us to trigger her.
It's really noticeable in Expert where they go all out, you will very often have Smoker's smoking you through spit, jockeys riding you into witches/spit/tanks, etc there in my experience. There have been a FEW dumb moments (a boomer sitting in an alleyway by himself, notably) but they were pretty rare in the long run.
That's how TF2's business model panned out, and it's proven that it works.
TF2 is definitely more simple than a lot of other FPS games on the market and it also provides a more unique experience than those other games so the kind of business model they have going for it is very unique and works with the type of game it is. That type of business model is definitely the exception and not the norm though, and in reality what L4D has done is nothing different than Baldur's Gate 2 coming out one year after Baldur's Gate 1's expansion or Brood War coming out the same year that StarCraft did (at near full price) and tons of other games from other companies doing the same thing. The only difference is people are holding Valve to a slightly higher level because they now expect free content often just because of TF2, when in reality TF2 is a different game made by a different group of people at Valve not really related to Turtle Rock Studios group at Valve at all (except they helped out a bit just because the TRS group is kinda small). Yeah, I'd have loved for L4D1 to receive some free content like TF2 but free regular content is not the norm yet - TF2 is the only professional game that does it to any serious degree. Other games that have "free" content tend to be funded by paid DLC or require a monthly free.
Not sayin' I like it, I'd obviously prefer free content often, but it's just not the norm at all unfortunately.