So far, I can only complain about 3 things:
1) there is a level cap of 20 (not there yet, but give me a full weekend with it)
2) no Trait at character creation
3) Tag skills don't get the 2-for-1 increase when leveling up
Really, though, it's wonderful. It's a revision of the engine Oblivion was built on, but it really has the Fallout feel. The first "bad thing" I encountered was a Radroach, then mostly humans, until I saw a Molerat (very classic), and Supermutants (which really look pretty cool). I was surprised when I was attacked by a security robot, wearing classis U.S. army markings (white star and numbering on an OD Green paint job).
Some of the beat up old homes could have a more realistic layout, but the rows of beat-down, burned, once-identical houses is pretty impressive in places where part of a subdivision semi-survived. Encumbrance is a issue you have to deal with constantly, which is pretty true to previous Fallout games. There are companion characters, but the only one I've met won't join me because my Karma score is too high (oh, you can view your Karmic title, but not your actual Karma number).