It of course to do with skill. That's why someone like Snitsky never made it. Not only is it about wrestling skill, it's also about a wrestler's ability to tell a story. An example of this would be Hulk Hogan, who you could argue was technically poor, but he could tell a story exceptionally well. It's also about who gets the crowd going with their charisma, for example a real top face like Cena will get the crowd cheering big time while a top heel such as Randy Orton will cause a stir and get the crowd booing. Additionally, it's about a star's commercial value. Cena has great commercial value to the WWE - he sells shed loads of merchandise and is the sort of star who can appear in movies and make WWE public appearances very successful. Furthermore, WWE stars have to be able to work well with their colleagues in the ring. Someone who is hard to work with in the ring and stiff will rarely last long due to other wrestlers not wanting to work with them. Overall, to be a top star in the WWE (or TNA even) you must combine nearly all, if not all of these characteristics (without being both face and heel, obviously). A mid-card wrestler like R-Truth or Vladimir Koslov only have a few of these and has them to a lesser extent (such as ability to get the crowd going). Any wrestler who has only one or two of these characteristics is almost doomed from the start and will end up being released pretty quickly.