It's always interesting to see what the fandom will pick up on and run away with as well as what they will completely ignore. Solas has an incredible depth of character, but a lot of what is out there seems to focus and expand on tidbits of information.
I don't mind the concept art hair because I make the distinction that it may possibly be how he looked before whatever it was that he did back in Arlathan. People go through life stages, their appearance changes over time. The concept hair works for me as a possible reflection of Solas' more hot-headed youth, while the bald head works for who he is now. I don't see one as better than the other because I acknowledge the idea that Solas has undergone some serious life changes to become the man we see in the game. But I can see the frustration with the part of the fandom that sees it as better than bald. When people start making such a black and white distinction about a character, especially one so well written, it tends to leave a bad taste in my mouth.
Personally, I like the fact that Bioware doesn't seem to pander to the notion that every LI has to adhere to some Hollywood notion of beauty, as much as I think some of the fandom would prefer.
With Solas, I think the added complication is that we have very little in the way of concrete facts on him. We do have a lot of half-truths and possible full truths (ie. the village he grew up in). But a lot of what we know about him is based on the inferences we make based on his banter with other characters, what he will give approval for, his conversations with the Inquisitor, his personal quests, and what Weeks has confirmed about him. And people can take the same set of information and infer different things from it based on their own life experiences and perspective. All of this gives you a bit more space to head-canon around Solas and how he became the person we see in game than say Cassandra or Cole because they come to the game with a more in-depth character history that the player is aware of either through the game or other Bioware novels/games/movies. Because of this, I think as far as Fic goes, Solas is probably the hardest to write, especially if you are hanging out in his head. I generally take the approach that there are going to be fics I don't like because they have a different interpretation of Solas than I do, but for the most part I can see where that particular view of Solas is coming from, even if it is coming from an inference that I find specious at best.
It will be interesting to see, once we have more definitive information on him, which head-canons turn out to be in line with what Bioware has down about Solas and his past.