A little more Otonashi background in the VN (or somewhere) would be helpful. We only really get to see a little bit of him from the train incident and his time with his sister, but nothing else.
Otonashi himself doesn’t feel completely bland to me, just… under-explained and out-of-place. The latter in part because there are capable people all around him, and that leaves him with pretty much nothing to do. Until he realizes they’re going about things the wrong way, that is.
I mostly agree with what everyone has said here, at least partially, so far. He’s somewhat bland, but has a more unique (in their situation) psychology (or as @IkaCZ put it, a “strong psychology”) than those around him.
As @Yerian pointed out, he’s different from other Key protagonists. Though, unlike other Key protagonists, he’s actually a nice guy. (Similar view to @Bizkitdoh’s.)
This didn’t really feel that way to me. I would think his interest in her was natural given that he is already a very out-of-place character, and what had happened throughout. With everything the SSS did against Tenshi, he kept on seeing her human aspects. For example, when they infiltrate “Tenshi Space” he tries to stop them once he realizes that it’s a rather ordinary place – that she isn’t in some way putting herself above the others.
Aside from that I agreed with nearly everything else.