I can see how that kind of fic could be offputting if you prefer a more aggressive Lavellan (we know Solas/Fen'Harel used to act like an arrogant hothead, who had enough knowledge and power to keep both sets of gods on their toes (Mythal made a point of showing he was her staunch ally in her temple, as did Dirthamen), and probably ran around as a wolf a lot, if his iconography is anything to go by, but I never got the sense he was deliberately predatory - dominant yes, predatory no, so it's a characterization thing. I suspect he hasn't changed as much as he thinks he has, he's just better at hiding it.). The wolf/halla dynamic (which appears to be basically femsub) isn't usually my thing unless it's done well, but I guess such writing scratches someone's itch in fandom, so more power to them.
I'm a bit late to the party (I still love Solas as a character and follow this thread sometimes, but it tends to move too fast for me to keep up). Anyways, I don't read a lot of those types of fics myself, but I do see where they are coming from. Personally, to me the appeal of the character is that while he is probably very capable of being predatory (if he wanted to), he chooses not to be that way. He's obviously intelligent, and cunning enough. He has a lot of experience and ancient knowledge that Lavellen doesn't have access to, so that means they aren't equals in this way. But rather than being a jerk about it and using it against her, he's perfectly happy to share his knowledge when she shows interest. He treats her with respect, as an individual, and as an equal in this way, despite whatever power imbalances may exist between them. Knowledge is power, and a lot of powerful people aren't like that, because knowing something others don't is an advantage. At the same time, though, he is concealing his true identity and keeps that from her...but that was clearly a decision he made before they met and became close, and so he's clearly bothered by it the whole time, even if he does feel it was necessary at the time.
As some of you all probably already saw, Weekes posted this article on Twitter a while back and I thought it perfectly summed up what it was I found appealing about the character:
http://t.co/nnVNfy33Rr