To a certain extent the SL server code has been re-written from the ground up, by the OpenSimulator coders.
However, do not expect big improvements. The current releases are measured in terms of hours up before crashing, not days or weeks; lag is just as bad as SL and sometimes worse, and all the constraints we all know about in Sl are there and present in OpenSim too. The only real improvements that I have noticed are the extra commands (which scripters in SL have been asking for for years) and the Hypergrid which allows you to teleport between any two Opensim grids (but not without its own problems). However, the software is still alpha, so maybe before they go to beta further improvements may be made.
What is needed is not new server code, but a complete rethink about hardware, architecture, and software. Entropia took a bold step when they did just that, and are about to launch their new world with a completely different platform underneath it.
How many more years of lifecycle are there left for SL's current technology?
Rock