For the most part I've really enjoyed playing Team Fortress 2 (I got into it recently after playing it at a LAN party). Push the cart (can't remember the actual game mode name, although if that is it, that's hilarious) is a pretty nice team based objective but just the sheer amount of... well... craziness. Now that's not a bad thing, in fact, that's what keeps drawing me back. But the classes can get to me (some just seem pointless). That's what I enjoy about Mann v. Machine. You can pick whichever class you want, but not all of them are beneficial to your team.
For instance, at the top of your screen you get to see what's coming in the next wave. Usually there's some sort of boss (aka a super Heavy, or a super Demoman) which have extra health and are twice or thrice the size of you. Annoying as heck but so satisfying to take down by teamwork. That's the emphasis that I enjoy so much about MvM. You basically HAVE to play as a team in order to proceed through the rounds and picking the right class is essential.
For maybe 3-4 hours of straight chaos I played with Broawesome, kid.matthew (yes same who I'm building with :D), ice, 007, myself and usually another random person. Having 6 people on a team instead of 4 (Left 4 Dead 2) really makes it a) more entertaining when we're all yelling at once (mostly something like: KILL IT! KILL IT! KILL IT! NOOOOOOO!) b) more unique, as I haven't really played a game that allows 6 players to co-op (usually it's either 2-4 or just straight up multiplayer) and c) and more dynamic as you each to try balance the team out and support each other with the right classes.
I will say however that once you get to the later rounds, you basically need straight up firepower to take down the bosses (and tank). I think at one point we had 4 Heavy's and I know we had 3 consistently several times in a row. A neat aspect of the co-op is that if you lose a round (there are 6 rounds total) then you go back to the checkpoint at the start of the round. Meaning any upgrade you bought for your weapons that round, you lose, but you keep the ones from previous rounds. It really is quite a fantastic way for co-op to work and really keeps it interesting.
If you have Team Fortress 2, or you have Steam but not TF2, I recommend that you get it and play it. I usually play on Saturdays if you want someone to co-op with but expect a few minute wait before getting into a server as you get put in a queue immediately but I digress. Thanks for reading! Cheers!