Depends on your use. For most desktop people, better hardware intergration, better program handling. For servers, more fine-toothed control over jobs, better failure control. For embedded people, an easier way to strip out parts. Of course, the best way to determine your improvements is to read the kernel docs.
For most desktop usage, the difference is noticable, but not major. 2.4 and improvements have linux keeping up with windows, so don't expect that 2.6 will be "faster".