I'm really puzzled by all the Arthur love, because that was the shallowest character of all.

I don't think Arthur was more shallow than any other character - he just had a very simplistic front. If anything, I think he was one of the more layered ones - prickly, but also surprisingly loyal (in a field where that seemed to be a rare trait - Nash sold them out, Eames was ready to bail on them once things got dangerous and made it clear that he'd have no qualms about selling Cobb to Cobol, and then there was Yusuf, who was bought with Cobb's share). He showed a gentle side with Ariadne, we saw Eames get under his skin, we saw him humbled and tested; the guy who wanted everything planned out was eventually thrown into a situation where he had to improvise. It made him, aside from Cobb and Fischer, the only character in the movie who got any kind of development, really - not counting Mal, as she was really just part of Cobb.

Then there were little moments like the smile he had on his face in the van while dreaming about fighting crooks in the hotel and his kiss scene with Ariadne, that showed that he was more than just a stiff.

And, of course, looking striking while kicking butt in zero gravity helps, too.

Also, he cocked up his job twice in the movie.

I can only recall the one time where his research failed and zanzando made a lovely meta post about why that might not be as big a cock-up as it first seems.