I had a ton of fun lvling paragon levels and trying to get gear good enough to play mp10. However once I got there, I realized there isn't really anything left to do. This is why I'm lvling a barb now.
I see a lot of thread about how casual players can't play inferno which is why they like the new crafting. If those players wanted to feel elite they can always just play hell mode. The content is exactly the same. The drop you get there will end up at the NPc just like the drop you get in inferno. Why don't people play there? Because it''s no fun when there is no challenge.
Inferno mp10 is an artificial ceiling. Once you are able to plow through mobs there, what will you do next? For me it's wait for them to raise difficulty again. But then everyone will just start qq about how they can get gear good enough to play the new lvl.
Before 1.07 was released anyone could have spent 20 dollars on gold and made a character good enough to play mp5. This means you could plow though mp2. For those of you that are going to say you can't afford 20 dollars for 100m, go get a job. For those of you that have a job, you should have spent the money. It would have made your playing time that much more enjoyable.
Now that crafting is here people will say now they can have bis gear with out 1b gold. Just realize that getting that gear is getting closer for you to switch you "I'm poor"threads to "there is no replay ability"
Tl dr. Most fun in d3 is finding gear to play all content. Once you get there you are done. Don't be in a rush to get there.