I think Morden is a great character. That said, the most character development he ever gets doesn't even happen on screen--it takes place in the novels--and that's a shame. He's mostly a cipher on the series. Which is creepy and effective, but doesn't lend itself to complexity.
The same could be said for Clarke, who never is anything more than a symbol. I never thought the anti-alien sentiment was dramatized very convincingly. It was too easy for our characters to overcome their xenophobia--and most of them didn't have any to get over anyway.
The telepath story, on the other hand, is the complex part of the story that was just getting interesting in season four (and somewhat in season five). A shame it couldn't have been more fully explored (or, to address Byron, better explored).