Well I'm probably going to repeat what everyone else has already said (and said much better than I will lol) but here goes:
In no particular order...
1. His belief in the Order. I think the Order, and his feelings about Duty and Honor, are a big part of who he is. Strip these things from him and you'd have some real conflict of character.
2. His religion. I am not a religious person, but I find religious characters who are written with respect to be very interesting. I like writing religious characters myself.
3. His fear. His fear and trauma have become huge guiding lights in his life, and that is very problematic. Fear rarely makes you do rational or kind things.
4. His introversion. I like thoughtful, quiet characters who feel more comfortable handling things by themselves (such as taking Wilmod out into the wilderness alone). Often these characters think they can handle more than they really can though.
5. His compassion. It does exist! He has compassion and respect for Aveline and writes Hawke a very respectful letter, simply so that they might work together to save the career of a good woman (I think is how he puts it). His fear is often as war with his compassion, though, which makes for very interesting situations (i.e. what to do about Keran).
6. His loyalty. He remains loyal to Meredith for as long as he can. I think we'll see an element of this loyalty when he comes to work for the inquisition. He's a very duty bound individual. Again, this is ripe for conflict and character development.
7. The respect he commands. The other templars and a few of the companions clearly respect and fear him. That's perhaps why it's so jarring for Meredith to call him a "boy" in the final scene.
8. His shyness. I guess this could be related to his introversion, but I find shy characters cute, whether male or female. Aveline's shyness about Donnic for example was just too precious. Bonus points when the shy character is very stoic, strong, and capable, i.e. Aveline or Cullen or Fenris. Too cute for words.
9. His appearance. I just like it for some reason. I'm not necessarily attracted to it, but I like his curly hair and his neatly controlled facial hair and just the way he appears to hold himself in the Last Supper shot.
10. His voice. Honestly, I don't totally love his voice, but it's become a part of him now, and it definitely wouldn't be the same if they didn't ask GE to do him!