Let me rephrase. "Fun" is of course subjective. But generally speaking, I think the VAST majority of players don't find the following fun at all:
Rolling a 1 for your HP when you level up.
Having to level down your PC simply because a monster TOUCHED you.
Having to sit out the entirety of a two hour combat because you failed a save in the first round.
Having to sit through an hour of combat before you can take another turn because each person's turn takes 20 minutes because of all the iterative attacks involved.
Losing magical gear you slogged through 20 levels to get with the casting of a single spell (Disjunction), or because a monster (Rust Monster, Black Pudding, etc.) touched you (again!).
Spending 45 minutes calculating all the parties buffs and stackable bonuses, only to have a bad guy drop a Dispel Magic or a Disjunction in the first round and then it takes an hour to recalculate everything.
And for DMs, I think the vast majority don't find the following fun at all:
Having players Wind Walk through an entire dungeon you spent hours creating.
Having players Scry-Buff-Teleport into the BBEG's headquarters and cutting him down in his sleep, thus prematurely ending a campaign you spent months building up.
Wanting to use a vampire as a classic villain only to end up with a near TPK and a party that is now about 10 levels too low for the rest of your epic campaign.
Having players who prefer to simply create a new character from scratch (and thus throw away pages of campaign backstory and history) because losing Con or a level just sucks too much.
Spending hours painstakingly advancing a monster or NPC only to see it die in one round.
And last but most importantly, having to create house rules or fudge die rolls to prevent any of the above from happening because the system is just fundamentally broken.
4e will be a success because it fixes these issues and thus will be easier and more fun to play than 3.x was for the vast majority of players and DMs.