At ss, yes, I'd look for an upgrade. Bartlett's 1 hr takes him to 12 on his career, so he gives you absolutely nothing in terms of power. He doesn't score, doesn't knock in runs, the only thing he gives you is a non-disastrous avg and a handful of steals. I'd rather have Andrus, who has more upside in terms of steals and hits in a lineup with more potential (although he's also hitting at the bottom of the order, so he's not a slam dunk). I wouldn't worry too much about Zimmerman, yet. He's been disappointing, but it's important to remember how young he is, still.
You have 3 top 2b, so I'd try to trade one of those for a top ss. Maybe even pair one 2b with Mauer if you can get a Reyes or Rollins in return (you may not find a taker, but it's worth a shot). Your other 2b can carry the load, and you have Hart for the util spot. Your starters are good, but if this were me, I'd consider dropping Thome for some more depth at sp. The relievers are a solid group. Overall, good team, and could be very good if you can make something of that logjam at 2b.