1. Better RAM management, compressed memory coded into OS X to prevent bottlenecks and slowdowns. Limited RAM goes further.
2. Better CPU management, doesn't run as hot and buffs out up cycles with intensive processes.
3. As mentioned above, still receives security updates.
4. Improved battery performance overall, due to CPU management.
With that said, you'll be fine on 4 gigs of RAM if you go to Mavericks.
I use a mid 2009 macbook on OS X Mavericks and it runs great, it was running much smoother than my GF's MBP 2011 on Lion. I think this is mostly due to a SSD vs the 4gb of ram upgrade (she has 4gb of ram).