Versus was the first game mode.
Problem with versus is you need to know the maps to play as infected - it puts alot of people off since whoever knows the map better wins.
Try
http://www.l4dmaps.com/
For L4D1 maps, if you haven't already. Coal'd Blood is great, Death Aboard quite good but could be better on level design, Death row is good, Night Terror is stupidly difficult, but has some nice themes (like Moria) Suicide Blitz is well made, but the rescue map is stupidly simple.
I tended to forget the crappy ones. Want to play the mushroom kingdom map though.
Melee weapons are great against hordes.
BSP remains the same AFAIK, I think what changes is the shader format. Stuff that gets broke appears as shiny silver - the sphere mapping.
Given the renderer change I'd guess a shader change is probably a given. But haven't done enough with source to know how flexible its shaders are.