Here is a suggestion that should upset some people
Under the current system, we get no xp for using skills too far above the mob we are fighting, and -apart from healing- game mechanics tend to stop us getting xp from mobs too far ABOVE the skills we use.
Why not make it official ? You dont get to claim any xp if the skills you used werent powerful enough to affect the battle.Perhaps if you are > 100 levels below what you are fighting, maybe you should find something else to harass.
If you are using mixed skills, then xp would be based on the highest used, and if it isnt good enough...well, unlucky. on the other hand, hitting with an ele spell at level 150 would allow you to gain (split) xp on your lvl 70 affliction, against a lvl 200+ mob - same as it works now - but to gain pure affliction xp you would need to hunt mobs below level 170 (example only).
If you kill something solo - obviously you deserve full xp for whatever skills you used, and in a team, it shouldnt prevent you from getting xp from skills in range of the target mob even if you do land a low lvl skill hit as well (eg using a low "fear" to drive off a jugula add, giving you time to nuke/heal )
This is in response to the suggestion(?) that players will get xp even if they fail to hit/mob resists all attacks.