Well if you find the base unstabile - get a round base for it
- say from Pat Warner.
For progressive depth cuts, there is a depth stop turret with
three stops you can set on the 621 (and other plunge routers)
for progressive depth of cut cuts.
As for the depth stop rod slipping - the 621's plunge is very
smooth and the return spring isn't overly strong - so you don't
have to really slam down on it to get it to move. I don't understand
why the rod should be slipping and allow for deeper than desired
cuts unless you really slam it against the turret stop - or really
really really lean on the router while routing. Did you by any chance
put oil, grease or WD40 on the shaft?
As for the Twist To Lock handle arrangement, I like the fact
that I can retain control of the router with BOTH hands and
NOT have to reach for a locking lever.