I LOVE the group dungeons, they are fun, challenging at times and bring people together. Currently though I rarely try running dungeons because a)Since I only get the quest once I want to enjoy the story and plan to do it with friends or b) I already have done the quest and wouldn't get anything out of it really.
My solution to alleviate this issue would be to make those group dungeon quests dailies. Now you could easily move the skill point reward to be bound to the achievement to the corresponding quest and then from there you could adjust the xp reward for the quest based on the lvl of the lowest leveled group member. And you could level adjust the mobs that way too so if I wanted to do Fungal Grotto with other level 40's it then becomes a level 40 dungeon. Still just as challenging (if not more) while providing a viable alternative way to level.
This idea solves a few problems people have with the game that I know about. If you want to level consistently you are strong armed into questing throughout the zones. I love the questing in this game and have no problem with that though I think those that can't (or won't) enjoy it shouldn't be in the current situation. Secondly, it makes the game more fun because of the level adjustments of the dungeons. If I didn't do Fungal Grotto at level and only got around to doing it at say level 20 my opinion of that dungeon would be way different than if I did it at level 12-15. Same thing could be said for all dungeons.
So this idea, if implemented achieves 3 things. 1) It can not only keep the content challenging, but it can also be used to still help lowbies through it . 2) Provides another way to level for those demanding alternatives. 3) It would add a lot more replayability to these dungeons. I have done the dungeons I can at most twice (once for me and the other to help out someone when I can). With this implemented I would be fine with doing all the dungeons once A DAY and most importantly HAVE FUN doing it. And having more fun is the best reason of all.
Just think about it Zenimax.