I can see why people would hate on this game at first. I tried talking a few people at work who swear by online games like MW2 into playing, and they said they hated it for the same reasons oftheheavens had; not realistic, kept getting killed in one shot by players who it seemingly takes an excruciating amount of time to kill.
That's the problem, though, with current online shooters, and it all started with Halo's "shield recharge" mechanic. Many gamers today are conditioned so that if they spend enough time not getting shot at, their health is automatically restored. But in TF2, that's where getting to know the maps come into play... knowing where the location of health packs are, knowing where the rest of your team is (and the likely Medic that's with them).
I pretty much started out the same way, disliking the unfamiliarity, but I guess the reason I kept playing was that it fascinated me -- it fascinated me because everyone seemed to know what they were doing, and that there wasn't a single "favorite" class that people were abusing to get easy kills. Even on 32 player servers, with 9 different classes, I never see more than 2-3 of any class on a single team.
And much of this is thanks to the sheer number of counters there are. Snipers can kill anyone with a well-placed headshot, but spend a lot of time in their scope and can't see anyone coming up beside them, and are hard-countered by spies. Spies have a timed cloak and are able to disguise as anyone from the opposite faction, making backstabs very easy, but can easily be discovered by the random Pyro spamming his flamethrower. Pyros are great at ambushing players around corners and the damage-over-time effect can be a real bother to anyone not near water or a medic/medkit... but they are weak at anyone attacking them from a distance.
There are plenty more, but these attributes are exactly what keeps me coming back to the game. I never get bored because I'm constantly switching classes during my respawn timer to make up for a team deficiency or because I simply want to get back at that Sniper that keeps picking me off.