If it were a decade earlier and this was 2004/2005 instead of 2013, you'd be asking the same question about Batista. Instead of The Shield's eventual breakup, it'd be Evolution's, and you'd be wondering how far someone like Batista could go on his own without guys like Triple H, Ric Flair and even Randy Orton there to speak for him any longer. But I'd have to say he did pretty well for himself. He became the biggest star other than Cena that the company has seen since the Attitude Era, and now we're already at the point where we're mostly excited at the prospect of him returning for a short stint at Wrestlemania this year.
Roman Reigns can go just as far, arguably further considering he's getting the proverbial rocket strapped to his back at a younger age than Batista did, and will thus have more longevity on his side.
As for his mic skills, that won't hurt him really at all since he has other tangible qualities to make up for it, like immense strength, look, presence, charisma. There's been guys before who became big stars despite not being the strongest on the mic - Jeff Hardy, Brock Lesnar, Randy Orton, Batista, Goldberg... Plus, he doesn't really seem that bad to me and there's always time for improvement.
