You have mentioned it and so have many other posters that Sheamus has 'improved' both on the mic as well as in the ring. Do you feel that main event is the place to improve? By the time you reach the main event you should be a finished product. Sheamus wasn't exactly that. The fact that at this point Sheamus and Barrett are comparable strengthens Barrett's case even further because Barrett has been here for a lesser time than Sheamus.
Also while you are correct in saying that Barrett is yet to prove himself on his own you must understand that it is pretty difficult to play the leader of a stable too. Barrett has accomplished that right at the start of his career.
I can agree with this. The main event is not the place to have someone who needs to improve. However, look at hte current storyline Barrett's involved with. Can anyone claim that Nexus isn't at the very least, really close to the main event? Hell, Barrett's been in a a very high level match in every pay-per-view since Summerslame. And, let's look at that, for a minute.
Summerslam: Nexus v. RAW (Final match of the night, pushed as a main event.)
Night of Champions: Six Pack WWE Championship match (Last match, definite main event)
Hell in a Cell: Barrett v. Cena (Late in card, after Raw's WWE Championship match. Treated as a co-main event)
Bragging Rigths: WWE Championship match (Last card of night, main event, also combining TWO seperate stroylines)
Survivor Series: WWE Championship match (Another main-event level match, probably late in the card, again combining two seperate storylines)
So, any complaint that you have about Sheamus being in the main event before he was ready is more than true about Wade Barrett. The problem is, while Sheamus stepped up, and improved heavily, Barrett has done nothing of the sort.
And, as far as being the leader of a faction goes, exactly how is that hard in wrestling? I can certainly agree with you that, in character, it's difficult for the charracter Wade Barrett to keep order. However, Barrett, the actual person, only needs to portray the character that he's given. And, frankly, he's only marginal at that.