"...he’s probably one of the least problematic and put-together of your squadmates."
That probably sums up why some people find him uninteresting. Add to that the fact he's human and not an alien, and he's working an uphill battle despite being a special biotic. Personally, I find Kaidan among my favorites. His story and development are just as interesting as any of the others. His demeanor gradually thaws during the hunt for Saren [ME]. He relaxes from reserved and aloof, to sharing his personal opinions with Shepard in private, and making wryly humorous commentary with the others in public. His refusal to join Shepard on Horizon [ME2] is quite understandable. After catching wind of who-knows-what-kind-of-rumors spread by TIM, seeing Shepard alive again had to be a horribly joyful reunion. His shock, anger, and resentment are normal reactions since Kaidan had *two years* to come to terms with losing yet another person he let himself love (even if it was basically only 'one night' for the commander). That said, he stuck to his ethical guns which was the right thing for him to do at that point in time. Much later [ME3], when he's had time to recognize Shepard again, he eventually agrees to rejoin the crew, because he's finally reassured he's doing 'the right thing'. Kaidan has just as interesting a character arc as any of the others, it's simply much more personal and not as flamboyant as any of the aliens' tales.