I think you should get more XP for soloing a quest or doing it with fewer people. Just like in PnP, exp for an encounter is divided to all the characters.
Just disable it in quests that are known to be too buggy or easy for some classes to solo and impossible for others.
Take into consideration the casual player who logs on his lvl 15-20 toon and there are no groups to join for several hours. I've been in this situation several times, no groups to join and nobody joining if I make one; which I rarely do because I don't know the dungeons so I can't lead.
So to get anything done I end up soloing a quest, sometimes spending several hours and only getting 5k exp or so. The achievement feels rewarding but the xp doesn't, and I think I deserve 25k xp for being the entire party.
Some additional crazy suggestions:
Also to make it more fair for various soloers, the game could calculate your characters challenge rating or effective level based on what gear you have and then give you xp based on that.
For example a level 14 with the most expensive gear would be more like a lvl 17-20 casual player and can finish the quest quicker.