Kaidan has a small but devoted fanbase of mostly female fans. The fact that he's hated so much probably causes some people to never admit openly that they like him, and I know that I didn't admit I liked him online before ME2 came out and he got a support thread on the Bioware Social Network, because I didn't want to argue with haters.
That being said, I like Kaidan because he is "just" a soldier who is dedicated to his job, and loves what he does and does it well (if you don't autolevel him and use him correctly!). Kaidan doesn't brag about his past, and he doesn't complain about it, but he will tell you about Jump Zero to illustrate why he is worried about Shepard, and the status of the current mission. He has always been reserved, and held back when it came to his powers as a biotic, but on this mission, when it really counts, he will give it everything he has. He is also the most "paragon" party member in ME1, and I like to play do-gooders, and appreciate the fact that Kaidan is very stable, responsible, and decent. Add to that some nice looking pixels and a great voice actor, and I was hooked in the first game.
Bear in mind that I'm a female who romanced him in the first game and stayed faithful. I don't expect everyone to like him, and I wouldn't expect every female player to want to romance him either, he's a very specific "type". Just wanted to give my own perspective on why he's popular with me.