Both games has some differences, you can find some stuff here:
Basically, L4D1 has a darker atmosphere while L4D2 has brighter maps and textures.
Also, L4D1 has a specific map (map*_vs_*) for versus and for coop, while in L4D2 they just one.
Some maps have different paths and models because of a balance perspective, since L4D2 brought some new SI, weapons, etc. (like No Mercy finale is open air in L4D2 and L4D1 has walls on the second floor, radio area)
L4D1 doesn't have melees so IMO is harder, causa melees are overpowered in L4D2