Injuries: The knee injury, though it may have lingering effects, was one event. He doesn't have a long injury history. We will certainly have to watch out for the knee in the future, but his health has looked very good this season...any setbacks would've been very disconcerting and put a position change into further scrutiny.
Power: The kid is only 22. First of all, the Twins are damn happy enough that he's been healthy nearly all year long. Getting back from that knee injury was the plan this year...and to be reasonably productive. Well, he's hit around .300 with an OBP in the high .300s, and a K/BB ratio that is near one. That is unheard of for a young player such as himself. His SLG has gone down significantly since last season, but he only hit 9 home runs in 3 Minor League seasons prior to last year's homer surge.
What last year's power numbers showed me, IMO, is that he is very capable of hitting for more power, but just has to learn how to do so. Power is the last tool to come, as it requires the body to fill out more, as well as needing to learn how to properly lift the ball. Mauer isn't exactly a small guy. I believe that as time goes on, he's going to be capable of hitting 30 home runs. It's not a guarantee of course, but the Twins' front office was hoping for 35-40.
As for now, he'll provide a high average, some SBs, and good R/RBI numbers from the catcher spot. He's a top-5 C at the very least. I foresee the #1 catcher spot in his grasps once he adds some power to the arsenal. I may be higher on Mauer than others, but he is just incredibly talented.