I don't think so. I think I remember specifically reading in the manual that the only difference is how much damage the infected do to you.
Regardless...This game is uber fun!
I can guarantee that infected do more damage, you do less damage or the infected have more HP and friendly fire is much worse as difficulty goes up.
My primary reason of dying in Expert mode is friendly fire, the infected hardly ever hurt me... (Except for the Tank sometimes.)
Taking a shotgun hit in Expert mode is around 50HP, it's barely a scratch in Normal mode.
Here's a list of bugs, you might want to know them in case something weird happens on your server.
http://forums.steampowered.com/forums/showthread.php?t=751344
I've already faced the Jumper spawn exploit, it's mighty annoying.
Switching between weapons reset the melee attack animation, so you can strike lightning speed doing that, exterminating any horde quickly, making the boomer a joke...