This is gonna sound kinda noobish, but I don't know who Carth is, nor do I frankly care. I wouldn't count myself as a ME fangirl, but I truly liked Kaidan.
His character was kind hearted and warm, and he was trying hard to deal with his issues in a rational manner. They clearly put a lot of effort into making his character more human. Flawed, but essentially good.
I find the cliche "deep" characters to be far more "boring and whiny". Take Jack for example, classic revenge character arc, culminating in blowing up the facility she was housed in. Or Thane, deadly assassin trying to repent when he finds out he is dying. These characters are cliche to the extreme. You know exactly where they are going and you may want to tag along because it may be cool to watch, but at the end of the day you haven't gained anything.
With Kaidan, however, it feels (at least to me), as real a character as you can get in a game. He is just another person trying to live his life the best way he knew how.
On a side note, I did the romance with Liara as well in another save file, and in ME2 I romanced Garrus, but at no point did I feel any emotional attachment to them. And that's what it is all about, that is what Bioware was trying to accomplish, emotional attachment. If it was only about the ability to screw aliens and other humans, you would not need to put so much effort throughout the games to get those relationships.
