I managed to play 1 level in the demo without my laptop overheating too much (hardware problem I need to fix (but it could just be dust)...not L4D2's fault)
I felt the engine ran smoother for me than the original Left4Dead, so maybe they've made more optimizations on the engine, rather than adding anything too different.
It seems quite fun...I'll have to convince my classmate to bring in his laptop into Uni so I can give it a proper test run.
I like that it is set during the day as well...too many zombie things are done at night...and well, if zombies did invade, the day wouldn't disappear all together, now would it?