Ok this is my personal opinion so I can't speak for anyone else.
He is too slow and plodding in the ring. Often looks lost when doing moves. Doesn't sell, but then again they didn't book him in a position to sell when I was watching WWE (he was doing the audience challenge back then). When he talks it sounds unconvincing. It's like he is ACTING, instead of BEING. Then there's your point of him being bland. In a tag team, they can cover those weaknesses by having his tag partner do most of the actual matwork, which he lacks, and then tag him in to do a few power moves/Masterlock and you got an effective team I think. Hell I thought he did better in that temporary team with Carlito than he did on his own. I know that not too many wrestlers can be as good as Chris Benoit or Eddy Guerrero but there IS a developmental territory for a reason. Put Masters back there and train him more so that when he comes back, he won't be so clueless in the ring.