Tobold wrote an interesting piece yesterday challenging the idea that playing in groups should be "harder" than soloing. Of course this raises the important questions of what we mean when we say a game is "hard" and how much difficulty is a good thing anyway?
Part of the problem is that we mean different things by the word "hard"; it can refer to something that requires either a great quantity of effort or particularly high skill and the two aren't necessarily comparable. Who's to say whether hauling coal is harder than quantum mechanics?
Older MMOs were hard in both senses. For example, EQ1 requires both skill to raid and a very long grind to get to the point where you could. In an attempt to streamline things, more modern games such as WOW have reduced the quantity of effort required to get to the highest level content, without necessarily reducing the skill requirements at the high end. It's telling that original WOW design team was dominated by high-end raiders from EQ - Vanilla WOW was aimed at that audience, with its emphasis on getting to raiding quickly. This philosophy reached its logical conclusion in Guild Wars, where it's possible to pay to skip the levelling stage completely.
The problem with this approach is that whilst you've reduced the amount of unchallenging content the high-end players have to plough through to get to the end game, you've also reduced the amount there is to do for those who either don't want or aren't able to do the raid content. Every time you make the game start quicker for raiders, you make the game end quicker for everybody else.
Consider the extreme case of a game where there is no levelling content, just one incredibly challenging boss to defeat at the end. A fight so hard (in the skill sense) that only the likes of Paragon could ever defeat it. How much fun would that game be? Now imagine the opposite extreme - a game where you could "win" by killing 28 bazillion boars in the style parodied by Southpark. Not much fun either.
A balanced game design needs both kinds of "hard" to be fun, but it also needs them in the right amount. Make your hard-as-in-skill bosses too hard and people will give up on raiding through frustration. Make them too easy and people will ... errr ... give up on raiding through boredom. Make your levelling game "kill one boar" and all the solo/small group players will run out of things to do and leave. Make it "kill 28 bazillion boars" and the raiders will give up through boredom before they reach the endgame.