So in my PbP experience, all the players were one level, but here in L4W it seems that I'll have alot of different levels. What advice can vertern DMs offer to help make multi-level adventures?
In my experience +/-2 levels from the target is ok, more than that is a stretch but even someone 3 or more levels below can contribute effectively if the character is at all optimized.
I tend to recruit in a 4-5 level band (my latest adventure was for 10-13). Then once I find out the levels of the PCs that are playing. I tailor the encounters for that party.
For example if I budget an AL+1 encounter into my adventure and my party is levels (10, 11, 11, 12, 12, 13), I'll add the +1 to each of those levels (11, 12, 12, 13, 13, 14) and then take the xp of a monster of that level (600, 700, 700, 800, 800, 1000). That total (4600) will be my budget for an AL+1 encounter.