It's not even just the fact he doesn't have a lot of opponents, but the fact that the creative team puts absolutely NO EFFORT into producing any sort of storyline for the mid-card. Even The Shield doesn't really have a storyline going on... they just beat people up and look badass doing it, but that's it. Nothing for people to sink their teeth into. All the creative ideas for storylines, if one ever comes up, is put into only main event feuds. Otherwise the storylines suck, are used only for veterans, or don't exist at all. It's just Guy A vs. Guy B. I actually enjoyed Swagger/Del Rio when it first started for the fact there was something going on as opposed to just Swagger/Del Rio happening to be opponents for Mania. Swagger and Colter were rebelling against everything Del Rio stood for, about living the American Dream as an immigrant, and they were going to take the World Title to restore credibility to the "once great land". That was refreshing to see a feud not on the top level being given something like that for the first few weeks before they lost sight.
Of course that was a World Title feud, you'd never see anything like that involving the US and IC Titles nowadays, and it's why a guy like Barrett can't get any sustainable heat. I mean when he was given the chance to be the top heel in an interestingly fresh storyline, he succeeded and became the most hated man in the business as the leader of Nexus. He got a ton of heat. That's what Barrett is capable of, and it's entirely on creative that he hasn't reached that level since. Barrett himself can only do so much. It's not like he's been given Del Rio/Sheamus treatment and still fails to get heat (like Del Rio, and Sheamus when he was a heel). He has all the tools necessary. Excellent on the mic, has some great presence, has some charisma, and despite popular belief, he's capable of having good matches (against Sheamus, Kingston, Mysterio, Ryder, and even the match with Miz the night after Mania was pretty good), and even great ones with the right opponent (which again, despite what some would think, isn't the case for everyone).
So hopefully WWE wakes up and puts the World Title on him before the end of the year. He should've had it back in 2011 at some point, but WWE fucked up all his momentum in a piss poor attempt to Ezekeil Jackson over.
Edit: Great idea Truk83, but again, because it would involve putting the spotlight on two mid-card and below talents, it would never happen.