The rule of thumb for a bad experience in 3.5 or Pathfinder Organized Play (eg, Living Greyhawk, Pathfinder Society) was you had to have two of three things true.
1. Bad gm
2. Bad module
3. Bad mix of players at the table
A fun group of players and a module that is well written can overcome a poorly prepared GM stumbling through the encounters and box-text descriptions because they'll chew on the scenery, make their own fun and nobody will suddenly die by a GM mistake in the combat encounters or have the adventure derail because the PCs are competent and the module's organized assuming at least some people will be running it cold.
A good gm and a good module can generally adapt it in minor ways on the fly to the foibles of dysfunctional, incompetent or contentious PCs and give a pretty good experience anyway, drawing them into the game.
A good GM and good players will have a pretty good game, no matter what the author of the module did, and I've seen some doozies there. My favorite anecdote along these lines had a fairly normal adventure start an encounter with the gm just going "sorry about this" and having the following monsters attack our river boat....
Flying awakened (intelligent) squid barbarians, juiced up on potions wielding multiple axes in their tentacles. The table was so busy busting up in laughter that they almost killed us all, because we just couldn't take it seriously. My wife's reaction when I told her later was "Squids are SALT-WATER critters!" (we'd seen enough goofy stuff that THIS was what offended her).
Mostly you get mediocre in two of the three, but one of them good, and that is also pretty fun. (and if the players are mediocre, well, you're part of the problem. Start injecting some fun!). As long as it isn't actively bad, most gaming is a bit like a good meal or sex. Even when it isn't great, it's pretty good.
Now in most games the GM is the author of the story, so a bad GM can kill a game pretty quick. Fate doesn't have the crutch of pre-planned modules, because it doesn't lend itself to that (it has scenes that after the initial hook are pretty much determined by player actions in the first scene - at best the GM has some milestones and stuff he kind of wants the PC's to interact with, but in the end they drive things much more than D&D/Pathfinder). This means the GM also has to be an author, and so do the players. If either side of this equation breaks down, Fate can be pretty stale. (see my other posts on the difficulties of running Fate in a convention setting, based on my anecdotal early experiences. It can be done, but it's harder if the GM is unprepared or inexperienced with the fate economy, or if the pregens are poorly thought out)
I was an experienced GM but not with fate when I ran a Codex Alera adaption for one guy good at Fate, one with passing experience and two newbies. We had fun because they were fans of the setting and I'm a decent GM, but I didn't give them enough aspects that fit the scenes well on the character sheet or in the scenes themselves, and we didn't use the fate economy very well (this adaption was like DFRPG though in that the non-fate add-ons were strong enough the flavor got through, even if the aspects were weak, which is probably why it worked)
I'm an experienced player, but had an inexperienced GM and a tired, cranky table using stock Spirit of the Century characters when I had a bad experience playing. I didn't understand my character, ran dry on fate points and got taken out in the firs encounter, got no help from anyone on the table using the aspect mechanics in terms of compels from GM and other players, was playing a social pilot in a scenario mostly underground and with nobody to talk to, so had little help from the skills part of my character sheet either.
I salvaged that game experience for myself on a break by just getting my head right and loosening up and looking for ways to chew on scenery to have some fun (which in Fate, tends to work well). In the climactic scene I used my "fear of flying" consequences from the first encounter to hose a bunch of flying enemy psychic things that had latched into my brain, and the "spirit of freedom" high concept to judo the BBEG who thought that me letting him into my head to do the fear thing would get me to become a thrall.
Running DFRPG here is really helping a lot to have this all sink in, although I'm not totally clear on the pacing of the fate economy. My players are mostly accumulating fate points while coping with various challenges, and we'll see how well I did when they reach the climax of this milestone - I've got that worked out, but I've got a lot less feel for how it'd go than in a normal game, given how compels and invocations can suddenly shift the situation. One really nice thing though is that the city building effort gave me a ton of stuff to work with when responding to their actions and looking for conflicts and challenges. The world building I usually have to do to GM my own scenario is mostly covered by the Dresden setting + city/character building, the rest fills in during play.
