Shelton is one of my favourite wrestlers and although it's disapointing to see him job, the reasons are fairly obvious:
1. He has no mic skills or charisma, meaning that none of the kids will get behind him. However, this could be remedied by bringing a manager in and Lashley is getting a fairly big push despite his lack of mic skill.
2. He annoyed someone in a powerful position (as mentioned by mabuza). Up until mid-2005, he was enjoying one of the longest IC title reigns in WWE history and having some great matches with the likes of Y2J. However, he was made to see out the rest of the year jobbing because of a backstage incident and although he won the IC again (albeit for a much shorter time) and had brief feuds with RVD and Jeff Hardy, he never reached the level he did before.
3. He's stuck in a dead-end tag team. Remember in 2003 when TWGTT were dominating SmackDown. Well not only has the reunion stopped Shelton from ever potentially acheiving greatness, but they're even light years away from winning the tag titles.
I think Shelton needs to split with Charlie, move back to SmackDown and feud with the likes of MVP and Finlay for the US title. They also need to give him a manager. Maybe Teddy Long; he's been SD GM for a long time now and made a great manager. He would definitely solve the problem of Shelton's mic skills.
Although it pains me to say it, I don't think the future holds anything spectacular for Shelton.