As Grimiku pointed out in the an [url="http://us.battle.net/d3/en/forum/topic/7004458205?page=3#47"]earlier post[/url], one of the things we want to avoid with the reward structure for Monster Power is creating a situation where players feel forced to run on the higher levels, even if they happen to feel more comfortable at somewhere between MP1 - MP3. Ideally, the added challenge should be the primary motivation for how you select your MP level, with the bonuses simply being a really cool perk that you get in return.
On the flip side, though, we know that lot players currently feel "forced" to run on the lower MP levels because it's more efficient to do so (especially if you're looking to farm XP). That's not a great situation either. So, while we don't want to change up the entire reward system for MP, we do agree that the XP bonuses could be more enticing at the top end, and we're looking to make some improvements.
I don't have a lot of details for you right now beyond that, but we've definitely heard your feedback and are trying to incorporate it in a meaningful way.
Challenge? Pfft. You can switch to a defensive setup and survive in MP10 for 1 million. Tedious sure, but not challenging. It's just beating on a sack of HP for a while.
How to fix MP in a few easy steps:
The bonus XP/gold/mf stacks multiplicatively and not additively with any other bonuses you have. Presently someone running around with a Radiant Star Ruby and a Hellfire ring gets 166% in MP 0 and 326% in MP10... but it takes far more than twice as long to kill in the latter. MF/GF is even worse, and the longer you play the less a proportionate benefit higher MPs are. These numbers are also before stacks, which boost the aforementioned numbers to 241% vs 401%... yet, it requires far more than 60% longer to kill in MP 10.
Reduce the HP scaling to something more sane, as the main reason higher MPs are inefficient is because it just takes too long to kill there. If this requires more damage, or better yet actual dynamic changes so it isn't just the same game with different numbers then so much the better.
Absolutely no one is going to buy into challenge for the sake of it (old Inferno should have shown you this) but many will if it is worth it. If the MP system were working correctly the optimal MP would be the hardest you could handle and not always 0 or 1 unless keyfarming. And the hardest you could handle would increase over time. That could actually work. It wouldn't be forcing people to go in high MP if they can't handle it as then they'd kill so slowly the lower MPs would be more efficient. However, if it is possible for the higher MPs to be efficient, then the system works as intended. Currently, they really can't for anyone.
Edit: Oh and take the monster density of Inferno and then double it.