I'm finding that it's not overly serious, just a bit more restrained with the "wild and wacky" stuff.
Yeah, that's what I was talking about earlier. It seems like they wanted a lot more coherence, which is what made the second game good: it knew what it was doing.
To put it in perspective, the first game is like the guy who's kinda funny, but usually doesn't talk much and doesn't stand out from the crowd except for being a decent guy. The second game is the class clown who says and does crazy shit, but is appealing when placed next to the more stiff and serious people and can be serious and intelligent when he wants to be. The third game is the class clown who doesn't really get what's funny (except to him personally), so he just says and does whatever random stuff comes to mind; some people think he's hilarious, while others wonder if he's actually doing anything intentionally or is just trying to be "RANDOMZ".
The fourth game is someone who's still funny and nutty, but tends to specifically do the most audacious stuff possible that will get the biggest reaction.