Well, the base 13" mbp's processor is .26 ghz faster (which unless you're spending your days encoding movies with handbrake you wouldn't be able to tell the difference), the ability to take 8 gigs of ram instead of maybe 6 (which may make a difference in two or three years if you keep the machine that long) and, oh yeah, the sd card slot.
The biggest complaint about the shiny macbooks, besides lack of firewire, was some of the screens on the first release models, from October 08 to January 09. Members of the tech press, as well as some of their owners, were dissatisfied with the viewing angles and contrast ratios (ability to adequately display and differentiate blacks and other dark colors) of some of the screens.
The worst of these screens were the 9C89's. (OTOH, I find my 9C88 to be acceptable.) Recently, a few macbook owners reported improved display qualities with SL changing the gamma setting to 2.2 from 1.8. Apple did a stealth update in the first quarter of 09 by putting in better quality displays that were a significant improvement over the earlier ones.
And before you wonder "Gee, do I have one of these 9C89's?", let me say that I had one, and found it to be horrid. If you're not complaining about it, you probably don't have one.
One advantage the uni MB has over it's younger brother is the ability to take any 2.5" sata notebook drive. 13" mbp owners have reported problems with their machines after updating to 7200 rpm and/or solid state drives, and apple officially does not support any 3rd party drive on them.