I get that - the romance writing is a bit 'thin'. Not terrible, but there could have maybe been more to establish what he felt. I think it requires the players to pour a lot more of their imaginations into not just the physical aspect of the relationship - which is whatever - but the emotional aspect as well. I think the romance with Solas would be intensely emotional - the physical aspect being so negligible to the point of being meaningless to him. The game asks the player to fill that in with their imagined Lavellan which is kind of a lot. LIke you're roleplaying as the other party in a ridiculously emotional and intense romance.
As Mims said above - what he sees in Lavellan - on a basic level is that she's principled, has purpose, and is thoughtful. On a more complex level, return to the above paragraph - it's however you roleplay your Lavellan to be and how deep the relationship goes.
Final note - I do NOT think Solas was coerced into the relationship. Lavellan has no idea what he is and that he intends to leave. She flirts because she likes him. He responds. It's an adult relationship. It's how people work...
Personally, I'm all for this. I love headcanon. I love fleshing out my character's background, motivations, personality and quirks beyond just what's shown in the game. I see the game as the basic skeletal structure of the character, while my imagination fleshes it out and fills in all the details.
I also love imagining why my character and LI love each other, beyond what's just shown in the game. If it's all shown scene-for-scene in the game, the reason Solas claims to love "you, exactly as you are" would be the exact same reason he claims to love any Lavellan. Then it wouldn't feel like he loves your character for who she is as a unique individual; that he could love any elven female plopped down in front of him.
Solas mentions on the balcony, right before the second kiss, that you a wisdom and subtly he hasn't seen since... since before his deepest journeys into the Fade, and he'll say that few act with such understanding of the world around them. Since he also approves of you being kind and helpful toward others, I think Lavellan's wisdom and kindness (and confidence and surety) are the central skeletal reasons, while the other reasons not mentioned in game are all the traits that make your character unique in your mind.
All the unique personality traits and quirks you can headcanon your Lavellan having that other people's characters might not have (like a habit of chewing her lower lip, or tucking a stray hair behind her ear when she's nervous, or what have you) are traits that you can imagine he finds just as precious in your characters as the mandatory traits all Inquisitors must have just for being in the game. (Like that air of surety and confidence that everyone in the game says you have that makes people follow you, even if your Inquisitor secretly has no idea what s/he's doing.)
I like headcanon. I don't mind that the romance isn't fleshed out completely in the game, so I can flesh it out in my own mind.