I really like the challenge of proving grounds, but I dont like that it is so much easier with higher ilvl gear because of the increases number of sockets on gear and that I have to make a completely different set for it - and I'm not talking about healers only, same goes for tanks.
Doing them in actual 463-476ilvl gear is a completely different thing that with current 2-3 socket items and sadly the tank PG revolves a lot around kiting and letting your healer offtank some mobs, which isn't my idea of being a good tank.
Another feature I'd like is to be able to get back into the PG at a higher wave, starting at wave0 everytime just to wipe after 30+ mins at a similar point again is frustrating, It wouldn't have to be at every wave but like every 10 waves you get a new "spawnpoint", that'd be great
EDIT: Imo PGs aren't really the way to learn your role, but they can learn you SOME key elements - and I'd say for most new players the threshold to actually try and do it is quite high (with the increased difficulty due to less sockets on gear etc.)