One of the things that was really striking to me during the recent Rift retrial was the ratio of exp per mob kill to exp per quest completion.
At level 38, I was getting about 800 exp per kill with rested bonus (400 if/when I ran out) and about 3600-3800 exp for turning in your average quest. For the traditional kill ten rats quest, this means that a third of my exp came from the quest reward, a third from the mob kills, and a third from rested exp (a mechanic that I think is out-dated, but that's another discussion). Sometimes, you might get 2-3 quests in the same camp of mobs that you can complete simultaneously, which makes the situation a bit more favorable to the questing, but factor in travel times and Rift is a game where you can actually level faster by grinding mobs (especially with a good AOE farming build), rather than questing.
On the opposite extreme sits Runes of Magic, which I jumped into over the weekend for a double exp event. At level 52, a mob kill was ringing in at maybe 300-400 exp (doubled for the weekend event), and quest completions are worth closer to 80,000-90,000 (i.e. 200 mob kills per quest complete, as compared to 10). This steep curve exists in part because the free-to-play ROM has a cash store item for resetting the daily quest limit, and therefore does not want players bypassing quests.
(Interestingly, there's an argument to be made that bonuses to item drop rates are actually more significant than bonuses to mob kill exp, because the latter is such a small part of your progress, while the former gets you daily quest items that can be turned in for much larger awards when your cap permits.)
Where to peg this ratio is an interesting decision. In some ways, mob kill exp is almost incidental to other forms of gameplay - whether you're questing, harvesting, or just trying to cross the road, you will probably kill some mobs somewhere along the way. In that context, though, it seems especially odd for Rift - which was intended to showcase spontaneous rift and invasion content - to weight the incidental kill so heavily.
There's a balance to be struck between rewarding players for doing this content with a nice exp boost (though honestly I didn't find that I got much exp for minor rifts or even zone invasions) and not pushing them out of the level range for the quests they're working on. Then again, Trion will have some unhappy players if the coming AAXP mechanic strongly favors grinding non-elite mobs over daily quests, rifts, or other things that players actually do at level 50.