IMO, the perfect starter mountain bike (which, in my opinion as well as others, is the best "commuting" bike) is the Trek 4300. It's MSRPs for like $530, but I have never seen it for sale higher than $450, sometimes as low as $400. It has bontrager seat, wheels, and tires with the same shimano click gears that every other bike in this price range has. I put a new seat on it when I broke the post (used it for trails that it had no right being on) and would recommend it if you plan on have this long term. I had the caliper brakes (disk brakes are a PITA) which I upgraded after a while to suit my personal preference, there's nothing wrong with the stock brakes though.
The seat and front suspension fork are the only real weak-points on this bike and, if you don't plan on taking it on too hard of trails, even these will not be a major concern for you.
Rides smooth, light weight, tough as nails, and the price ain't too bad.