It hasn't really been mentioned since Diablo 3's RMAH announcement last August, but does anyone else remember Blizzard stating that we would be able to sell characters in Diablo 3? Seems to me like they have forgotten or placed this on the expansion backburner.
+Experience per kill would be an amazing stat if you were in the business of selling Characters. You could theoretically collect all the non-class specific items and create half sets of gear for power leveling. Imagine having +30 Experience per kill for your first 20 levels. Then maybe +120 for 20-30 levels, etc. With how often you kill monsters, this would be a huge bonus.
Then of course, there are the monsters that spawn in special events, like the glowing jar in the Cemetery Crypts. Even the weakest monsters will give you at least the amount of +Experience you have on your gear, so that makes farming weak monsters pretty idea for those kinds of builds. Lots of testing needs to be done (and will) for determining the best gear and farming locations for powerleveling characters.
If selling characters doesn't become a reality, then +Experience is going to be a pretty worthless stat. Sure, we'll take it if we have it, but we will sacrifice it any chance we get for more survivability.
Speaking of uselessness, think about how terrible +Experience gear would be at max gear level. Which brings up an interesting speculation point: imagine buying +Experience gear on the auction house in preparation for an expansion. Obviously, I have no idea if this kind of strategy would work, but you'd make out pretty well if it did!
Getting back on topic, let's say that Blizzard does allow us to sell characters. You could create a set of gear that would work for all characters, and use it on each class that you leveled. You would also need new sets of gear for different leveling brackets, such as 1-20, 20-30, 30-35, 35-40, etc.
Here is an example graphic of the kind of gear you could focus on for +Experience sets:
|Experience Gear Set|
- EDIT: Shoulders, Chest, and Helm too!