This is a very good combo, however, it can be improved in a very major way.
First, not only can you use a Quirion Dryad, you can use Titania's Chosen.  Variety is the spice of life after all, isn't it?
Titania's Chosen:  Urza's Saga Uncommon 2G
Whenever a player play a green spell, put a +1/+1 counter on Titania's Chosen.
Again, basically the same as the Quirion Dryad, as it fulfills the same purpose (gobs of +1/+1 counters).
However, one card would greatly increase the potential of this combo:  Aluren.
Aluren 2GG Tempest Rare
Any player may play creature cards with converted mana cost 3 or less without paying its mana cost anytime he or she could play instants.
Grante,d the opponent will be gaining the ability of the Aluren, but you can (ab)use it in so many ways.  Fleetfoot panther is only 1GW, so you can Aluren it.  And then use its bounce ability to bounce it back.  And play it again.  For free.  Again.  And again.  And again.  Eventually, you'll have x/x chosen or dryads, depending on how often you feel like bouncing your fleetfoots again.
And to guarantee they don't counterspell your panther, chosen, or dryad (Sorry Aluren), use Gaea's Herald. 
Gaea's Herald: Planeshift Rare 1G
Creature spells can't be countered by spells or abilities.
Not necesary, but it helps.  Along with Rancor and Armadillo Cloak so that your creatures trample, you got it made.