Newly-upgraded Mavericks machine hangs on startup
1) You have a backup, right?

2) Boot into the recovery partition: hold option before apple logo appears, select recovery hd.

3) Run disk utiity form the recovery hd, select your boot partition (usually macintosh hd), and hit verify disk. It will probably find errors on the partition.

4) If disk utility finds errors, hit "repair disk" in disk utility. If it succeeds, you should be able to boot normally. If it fails, reformat and reinstall. 

If disk utility doesn't find errors... try reinstalling without reformatting.