Every QB doesn't have a great route runner like Welker who is open virtually all the time. Now Brady doesn't either, so the Pats will have to find someone who can do it at least some of the time (maybe Amendola, maybe someone else) or else you'll REALLY find out how much you over rate Brady when no one gets open consistently and he starts struggling.
The question then becomes: Does Brady do what most other QBs do, and try to actually make plays, resulting in his INTs going up - or does he just throw the ball away a lot more and/or take a ton more sacks?
Bottom line: It's easy to succeed when you have a guy who is open almost every time and never have to make a play yourself. Most QBs don't have that luxury, but Brady has had it for some time now.
On a side note, Peyton is going to be awesome with Welker coming open underneath for him all the time. Welker just might set the receptions record. Then again, Peyton actually makes plays downfield, so he might not have to dump off to Walker as often as Brady did, so maybe not.