I have an older Gen 2 21 that doesn't have the drop-free magazines. basically, what is the difference ... are the mags just a little 'slimmer' so they don't hang up in the magwell, or is there some other difference?

Also, will the drop-free's work okay in my 21?

Drop free is a poor term. Glock never said certain mags were drop free or not. The tendency is for the older non full metal lined ( lined is c or u shaped inside the mag body.) to buldge when loaded and therefore stick in the well when partially loaded and the mag release is hit. They will usually drop when empty, but most teach to strip the mag out to be sure. Later mags have a full box or tube of metal lining that more likely allows it to drop even partially loaded. And yes all mag types are interchangeable between all vintages of guns

Thanks. A gunshop guy was looking at my pistol awhile back and commented on my older, non-drop-free magazines. Said the later versions were drop-free. I had never really thought about it, since i don't do matches or tactical work.