I am the primary GM. Learning and teaching the systems has always fallen on me. Now when I was younger and in high school and college, it was fairly easy to keep all of these different systems stright in my head. Then as I got older, and became a computer programmer, got married, had a kid. Now life is just far more complicated, and it seems that my head has less room for the extra.... drek.
I began getting real tired of everytime I wanted to switch genres, I had to relearn the rules and reteach them. The gaming group only has time for a one night a week session, and all we really want to do is play.
Now we primarily play D20 - because it is intuitive. We still play WW World of Darkness. I have enough brain power outside of Real-life to know the rules for apprx two systems and their variants. My players sometimes want to try other systems, but I say sure - you GM it and learn and teach the rules. that pretty much quashes it.
For me, D20 has finally grown into what I have always wanted it to. With all of the different options and toolkits and settings out there, I can pretty much use D20 in whatever genre I want. We don't use minatures, and play really cinematically. And enough sources to determine which autofire rules we like best, for example. So in the end, D20/OGL has more than enough to keep me. Very soon, my decade long WOD campaign will be over, and then I fear that WW will no longer be played. I wish WW would open the storyteller to Open Content. I think the influx on new blood would help rectify some of a great systems few flaws, and breathe new life into a great system.
If I could have one D20 mechanic that I don't have, it would be a leveless D20 mechanic. Something in which players are rewarded experience and spend it as they wish on whatever they wish. We are close with Mutants and Masterminds. The closest I have seen is the OGL section in Godlike, which presented the bare bones of such a system (by Mike Mearls). I have created my own system, which accomplishes this, but would love to see some other publishers takes to see how mine stacks up.
Razuur