There are a few things I would like to change or improve upon.
First, Alan Branch. I like the guy and we've done great things with him since reclaiming him from the Arizona scrap heap, but he is a pretty one dimensional player. He's very stout against the run but doesn't provide much in the way of pass rush. Ideally, I would like to see him replaced with a top talent, someone who can play three downs, is stout against the run, and can shoot gaps or collapse the pocket during passing plays. Branch would still be a great backup, though.
Second, Marcus Trufant. Tru is not suited to playing in the slot. His entire career was spent lined up out wide where he does quite well in man coverage, but at this stage of the game he is the fifth or sixth best cornerback we have on the team and I would like to see the slot covered by some of the promising young blood, or WT3 if he ever manages to get his body right.
Third, I would look to improve things at the left guard position. That spot is well served by McQuistan, but he is a relatively unspectacular lineman at a spot where our scheme calls for a mauler. This is certainly not a position of great need, but we could certainly improve here, and it's tough to say if James Carpenter will ever be a long-term solution for us (though to be fair, football is full of unknowns, many of which are beyond coach and player ability to control).
Fourth, I would LOVE to see some better production from our wide receivers. I'm sure some of you reading this might be wondering "WTF, why?", after the pretty good seasons we are getting from Golden and Sidney, but the reality is that their production can be attributed more to the magic of RW's legs, which causes coverage breakdowns to occur when our guys are absolutely unable to get open. I'd like to see us get a real monster at this position, a guy who will fight for contested footballs and come down with them regularly in one-on-one situations. We don't have that right now, and haven't since Mike Williams' 2010 season. Sidney Rice is probably the most non-physical 6'4" receiver I have ever seen. His hands are good, but he just looks clumsy when he runs with the football, often falling to the turf untouched after making a catch. I like Sidney a lot, he is a good player, but we could use a physical monster who can break games open. Tate has the physicality and game breaking ability, but doesn't get open with RW in the pocket consistently enough, relying on circus antics from his QB to start breaking the coverage down. Could Sidney succeed in the slot? I can't say, but I would like to see someone better lining up opposite of Golden.
Fifth, I would like to see us draft an heir apparent to Leroy Hill. Leroy has been good for us but he is getting long in the tooth at a position which is generally better served by youth. Drafting his replacement (perhaps Morgan IS that replacement already, dude's been a baller for us but is undersized and has been nursing injuries) should necessitate a second, third, or fourth round pick, provided the value presents itself. Imagine this defense if it had a dynamic Clay Matthews type who is a threat to drop into coverage, stop the run, or rush the passer. Our defensive scheme doesn't seem to ask a lot of the OLBs, which appears to allow them to freelance and make plays a bit. A dynamic OLB could be a game breaker on defense.
These are the five things I would like to see changed, more or less in the order I see as most important. After these five things (a couple of them are even a bit nitpicky), it becomes hard to find areas of weakness as this team has so many very strong pieces to it.