I'd quite easily say Ron Vlaar is up there.
Don't get me wrong, on his day he's an excellent defender and he had a great World Cup, but 'his day' comes once every 5 or 6 games and he's guaranteed to miss at least a third of the season through injury. He's become a liability for the team because of his shocking injury record, and this season has been outshone by a free-transfer in Phillipe Senderos and a 22 year old defender who hadn't played for over a year, Jores Okore. The rumours that clubs like Man United, Napoli and Roma are considering signing Vlaar in January for (reportedly) 8-12 million are incredible. I'd take 4 for him and consider it a good deal.
I consider Jack Wilshere pretty overrated too, though in recent years several people have expressed a similar opinion (notably Paul Scholes). I've never seen him contribute anything which justifies his consistent inclusion in England squads and a starting place at a team like Arsenal. Plus, like Vlaar, he's guaranteed to miss a significant portion of the season through injury due to his glass ankles. I could honestly name at least 20 central midfielders I've rather have in my team than Jack Wilshere.
Who do you think is overrated?