One of the common trends in MMO's is the simplification of the solo game. Of course, it is all being done in the name of accessibility. The challenge, and I use that word loosely, is put behind the velvet rope of group play. As someone who enjoys the solo game, this ends up making a lot of games just not fun for me.
My definition of challenge is simply having to think. In many PvE games, very little thinking is required to get to the level cap. Sure there might be a lot going on which appears to add complexity, but usually it is completely superfluous. I want the decisions I make to actually matter. I'd also like to have to prepare for a battle.
Crafting is a lot of fun, until you realize it is not necessary. It may shave 1 second off of the battle, but it isn't needed to actually defeat your enemy. It's nice to get a ton of abilities, but what is the point if you really only need two or three of them? Again it usually comes down to making the fight shorter.
Story is usually what makes PvE bearable to me, but what happens when the actual gameplay is not challenging? You set out to defeat a great evil, and he dies in 5 seconds to a couple abilities. That doesn't really match up with the idea of a heroic story.
There are a number of games which do try and give their solo players and challenge. Star Trek Online, for example, has difficulty sliders which affect all of their combat. Raising this actually made me feel like my choices mattered. Lord of the Rings has skirmishes which allow you to adjust their difficulty. Sadly, that does not apply to their epic quest-line.
One common suggestion is to seek out higher level content. I find that it usually doesn't help, it is either still easy or impossible. Combat formulas in most MMO's end up preventing you from really doing anything to a monster who is X amount of levels higher than you.
I've done group content in my time, including some raiding. I never actually found it challenging, instead it was just tedious. Dealing with other players is hit or miss, and most group content is just following a script anyway. Besides, I just don't have the time to do that anymore.
Is it too much to ask for some challenging solo content? That's why I enjoy PvP games like Warhammer Online and League of Legends. I still think there is a place in my gaming life for PvE though, I just need it to be interesting.