I honestly can't say that I've ever heard a huge cry of praise for Team Fortress 2. If not for me, none of my friends would have even ever heard of it, and none of them play it at all regardless.
I'm not sure about everyone else, but the reason I enjoy it as much as I do is because it gives a comic alternative to the online FPS choices, of which Call of Duty 4 and like games reign supreme. I used to play Call of Duty online for days on-end, but, after a while, it got a little old seeing the same gritty streets with the same exploded cars smoldering and the same soldiers fighting back and forth down said streets. Whereas, when playing TF2, I find myself laughing at some of the stupid comments the characters make, or the taunts that they have, or the way the character models react to dying, or what-have-you.
All in all, multiplayer gaming in general is all about finding something that works and going with it. I mean, you mention the fact that TF2 seems repetitive, and give the example of building turrets in the same place. Yeah, okay, but I counter with the fact that snipers in CoD4 will always go to their favorite sniping perch, or how StarCraft (or any RTS game, really) players will find a procedure to go through when starting a game and repeat it over and over again. When you get right down to it, multiplayer games are about repetition, and very few games try to throw wrenches into that formula.
TF2 does, in a way: they give you a choice of classes, all of which have a way to counter that choice. Engineer have a turret built up? Sneak in as a spy and blow it up, or shoot a couple of rockets at it as a soldier! Heavy mowing down your forces? Come in as a medic and heal them so that your team can fight back, or spawn as a sniper and headshot that guy! Demomen tossing grenades everywhere? Come in as a scout and outrun the explosions! And so forth. There are always options to counter the repetition that players fall into, which means you have to think on your feet and react fluidly.
Yes, I know that this can be said of things like Call of Duty 4 and other such games as well, but I don't mean to say that TF2 is better. Every good multiplayer game gives options to counter the opposition. At the very least, though, in TF2, you can think in concepts instead of details. "Soldier counters scout" is a lot easier to remember than "M1 Garand at medium-range, with grenades, counters MP40", or what-have-you. (Bad example, I know, but I hope people get what I'm saying.)
In a nutshell, basically, I think the reason people enjoy Team Fortress 2 more than other multiplayer games is because of it's sense of humor, it's balanced class system, and the way it tries to counter repetition in its gameplay. But, really, it's all just a matter of opinion anyway.