Sigh.
Why do people constantly expect everyone to defend their choices in video games? Isn't it enough that we enjoy them?
Okay, I put my hand up. Yearly Assassin's Creeds are something I look forward to and like. I am their market. Yay me.
Firstly, they always release on my birthday (usually on the day) so that's nice, but not much to do with the game.
So, it's a stealth game, but not the Sam Fisher "Gonna make you do this twenty times because you blink too loudly" kind of stealth game, but the "oh bullocks, they found me, better pull out my sword and be kickass" kind of stealth. You may notice Mr. Boss as my avatar. As you can see, I like this kind of stealth. Where you can be stealthy, but doesn't expect you to spend hours waiting for a guard to pass or piss or whatever so you can get on with it. Assassin's Creed can be nicely paced and you can get shit done in a single session.
2) The setting. Walking around Venice, Flourence, America, Constantinople and so on is fun. I enjoy it. It's interesting and different and colourful, unlike so many Gears-esque grey-brown marathons. You're not often on rails, so if I want to, I can climb to the tallest thing on the map and look around. Or, as I usually do, I can look down from the roof on my target and feel like Batman's ancestor. Next up we are an ex-Assassin in some artic place, and a group of French Revolutionaries. Well, that sounds fun. Guess I'll buy those.
3) Story. Yes, I like the stories. I like Ezio, Altair is a bit flat but he grew on me, and Kenway is 5 kinds of awesome with a sweet ending. Connor and Desmond... well, when you release so many games, you're gonna make mistakes. But if they've released 1, 2, 2.2, 2.3, 3 and 4, and they've messed up 1 of them, that's an 80% hit rate. That's better than Total War does. And even I can admit that Ass Creed 3 just needed to be a little more focused, and have a better protagonist, and it would have been a lot better. Nothing could save Desmond though.
4) Gameplay. The gameplay isn't about intricate fighter-esque combos; it's more fast-flowing than that. Originally, it was a game of "Wait for the counter" but now you have sneak attacks, a knife, a sword/axe/club/mace, guns, smoke bombs, jumping from above, ambushing from behind, poisoning them, using your allies to help ambush them, sniping them and so on, all the way down to "Sneak past them". Your combat is as fun and different as you're able to think of at the moment.
Finally, people just enjoy the games they enjoy. You don't like Ass Creed? You want hardcore stealth? Go play Splinter Cell, Thief, Styx or whatever else scratches that itch. For me, Ass Creed scratches my "Oh god, that was fun and it didn't take me 50 hours to finish" itch.