I just recently saw this video from Blizzard talking about the challenge level of Diablo 3.
Their promise: You will die. I approve of this message. One of the unfortunate things about Diablo 2 is that there are so few areas where there is any real challenge in play for a properly built character. On your first run through it can be a little bit rough but you quickly get enough gear that any new character blows the entire game up with ease barring the challenge zones at the end which require brutal grinds to activate. Their stated aim with D3 is to change that baseline and make it incredibly difficult for people to survive in the highest difficulty levels. Presumably with a lot of farming, trading and crafting we will eventually be able to defeat Inferno without much difficulty but I do hope that takes awhile. Having to find a good spot for farming while considering danger, kill rate and item quality is interesting and slowly getting further and further along is enjoyable.
One thing I suspect we will see a lot if things are actually as hard as Blizzard suggests they will be is grouping and characters taking specific roles. I know that I could build some extraordinarily tough Barbarians and Monks who weren't much good at killing and I figure those characters will be at a premium for quite some time. If you can bring along a legitimate tank who lets everybody else in the party just pour on their damage without running around it will be critical to success. This will also mean that it will be very advantageous to have a group of good players you can rely on for groups so that you don't have to try hard content either alone or with buffoons; either of those might be extremely frustrating or dangerous.
There is a new model going on for the Real Money Auction House too, which is that there will be a 1.50 Beta Buck minimum listing fee and 1.25 of that goes straight to Blizzard. This means that it will only be worth listing items of some reasonable value - the RMAH will not be flooded with junk to sift through. Also each person will be limited to 10 auctions which seems directly targetted at botters and such. Although a 10 auction limit will negatively impact the real players who want to play the RMAH for fun it will completely cripple those scammers and gold farmers who want to fill the RMAH with thousands of crap auctions like they do in WOW.
Blizzard seems to be really interested in providing an optimal experience for the player who wants to play D3 a lot and doesn't want to be inconvenienced by the traders, botters and other predators. In particular they want it to be as easy as possible for that Joe Average gamer to buy things so that Blizzard can get their juicy cut. It won't be ideal for me as I like playing the AH and being a trade baron but I am suspect that Blizzard knows that I will be playing anyway - and doing this might get me to buy multiple accounts to play the AH on, which is good by them.