Overall, I enjoyed Zombieland, but it felt like about a 45-minute movie padded out to two hours. They filled it out with long pauses, with set pieces that didn't always add anything, with uneven pacing, and with lots of gratuitous gore.
It's hard to be critical of the movie because it was very intentionally over the top in some things, winking at the audience, playing lots of meta-humor, and enthusiastically choosing not to take itself seriously. So any criticism risks the response, "but that was on purpose." When they went over the top on the gore, particularly in the first ten minutes, they went way over the top, pulled out all the stops, made it clear that they were doing it to make fun of the very premise of gore in this kind of movie. And yet it still went past funny into "okay, that's too much, now let's have something funny again" to me.
And that's how a lot of things in the movie felt. But there was still a lot of good stuff, enough to make it a worthwhile idle entertainment. The best parts were the characters and their interactions, and the main character's rules for survival (and how they kept coming back up). Those moments when they went over the top and spoofed themselves and did so in a brief form were also uniformly good, with some of them providing surprise laughs.
I don't know if they could have made it less uneven, given the premise, and the goal of appealing to an audience mostly made of people not much like me. I suspect that some of the over-the-top stuff would have been funnier to fans of the "serious" zombie genre (which I am not). But this wasn't just spoofing the genre (Shaun of the Dead already did that), it's more of a try at riffing on the spoof once the basic spoof has been done, and that's where some of the unevenness comes in, and that's why I don't think being a big zombie genre fan would have changed it nearly as much as you might think. I think it's just more whether you appreciate the "taking things too far" metahumor of taking a metajoke too far.