I believe his only weakness (that we've seen) is the fact he can get too aggressive. He's been hit coming in with some nice shots (Poirer and Brimage fights). Obviously, this is hardly a weakness (since this is a fight, you're going to get hit). He fights and comes in with his head straight up, it's part of his style (which works) but I think he *could* get clipped with this approach, but it'll take an elite striker (Aldo, Swanson, possibly Mendes,Frankie) to capitalize on it.
I think this is concerning going into the Siver fight. I'm a huge McGregor fan and I'm nervous (hopefully, I'm just being overly anxious because i want nothing more than Aldo/Mcgregor). Siver is a great striker, a lot of people assume he's a grappler because of his build. He also has one of the best spinning back kicks in the UFC. Not to mention, which I think is his best weapon going into Sunday, is this strange falling away, sneaky, weird, left hook, that could catch an overly aggressive Conor.
Other than that, the obvious is wrestling. He showed takedown defense against Brandao, which while impressive, it's still nothing compared to Aldo, Frankie and Mendes.
Personally, while I do think he can get taken down, I think he's too big, strong and too good of cardio to get rag dolled (like Frankie/Swanson) for 5 rounds. I believe his TD defense is good enough to not make him a total liability in the grappling department (he's a brown belt and he goes up against Nelson, who's bigger and an elite grappler).