This question came up on the Facebook Fate group ("What makes a good compel?") and I wrote a really long reply that I thought might make useful blog content.
The background is a GM who is finding that his compels are either too weak (he had a wizard who interferes with technology cause a bomb fizzle instead of going off as planned) or too overwhelming (one of their players has the trouble of On the Run From the Mob, and now the entire group is). Another time a player whose aspect is Curious asked a bunch of rude questions at a dinner party and completely cut off that source of information to the entire group. Another player's trouble aspect is "Missing an Eye." That should be all you need to know for my response to make sense.
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My rules of thumb for compels are as follows:
I start with a general outline of the adventure in mind. Not what they'll succeed at, but what they'll have to roll against or deal with. It's usually four to six scenes. (One per player in the game, usually.)
Then I try to come up with two possible compels for every scene, AND/OR plan two compels for every character across the session. I do this ahead of time so I don't improvise something that derails the story. Players who signed on to do a fun heist romp won't necessarily enjoy a thriller about being on the lam with blood on their hands. It's important to deliver the experience that the players signed up for.
Therefore, my usual measure of a compel is this: it should be something that extends the game by two scenes--one "oh shit!" scene where everyone's jaw drops and players have to suddenly escape or stay and put out fires, and one follow-up scene where the players go, "Great. What alternate approach do we do now?" Often a player will take a consequence as a result of what just happened, and everyone loses time if there's a deadline clock. (There should generally be a deadline clock!)
I try not to cut off entire avenues. Instead, the difficulty will go up: +0 to +2, +2 to +4, +4 to +6. The alcoholic can get drunk and offend the duchess ("Oh shit! Dawn! Get in there and do damage control!"), and the team will still get her information--but it'll be harder, you're less likely to succeed with style, and more likely to succeed at cost...and everyone on the team knows who cost them.
What this means is that a compel goes off something like a controlled explosion. Someone who's Wanted By The Mob will not suddenly be attacked by the entire mob. That would change the story too much. Instead, the problems should proceed incrementally: Someone at a negotiation thinks he knows our ex-mobster, and the ex-mobster has to lie convincingly or leave the room in a hurry. Later, the ex-mobster has to stay outside this particular club because someone might recognize him...and so the team has to handle this particular task without their most charming player along. (Damn their luck!) A guy from his past approaches the ex-mobster and blackmails him, saying he just needs to delay his group from robbing the museum until the mob has a chance to steal the same object (now the players pull their heist only to find their target is missing, and now they have to get it from wherever the mob has it: two scenes). Eventually, other players start to worry about their colleague throwing them off the scent, there's a confrontation, and the whole team agrees to work together to end the blackmailer somehow...now you have the whole team buying in, just in time for a satisfying major milestone that feels appropriately major.
You use the compel as a source of constant low-level acute tension. Try not to blow the whole dam. You want players to say, "Oh, shit!" but to recognize that the compel is fair and sensible. And you want players to smile as they accept the compel: they made their character an alcoholic, and now it's come time to have fun with it.
Note that your compel doesn't have to immediately pay off. A player who is unusually curious might open the wrong book and hear a voice say, "I Am FREE! And The Doom Will Soon Descend Upon You All!!!" You don't need to specify what it is; you've just given yourself permission to screw up the player's plans and story any way you want to in the near future...as long as it makes sense within the genre. What I'm saying is, you don't need to screw up the current story; you can open a dark and scary door to be dealt with later instead.
That reminds me: Try to stay true to your genre. If you're doing by-the-books urban fantasy, your players will be more interested in "Oops! I unleashed a curse!" than they will be in a realistic heroin-addiction plotline. If a player in a Buffy-style game is a heroin addict, it should be treated the way Buffy would have treated it: probably kept in check with a magical cure from a wisecracking hedge wizard. He could miss a dose, need a favor, mislay an important package including the player's regular therapy...but you're not likely to have a long painful psychotic break or cut your arm off like something out of Requiem for a Dream.
Anyway, that's how I do it: I try to plan a handful of compels ahead of time (I don't usually use more than one per scene, but I always have two available), and I think of them as having something like a two-shift impact (+2 difficulty to the current task) plus an additional complication (loss of time, loss of secrecy, loss of social status, loss of hearing...). Remember, the player is getting a fate point for this, so the compel ought to be doing roughly a fate point's worth of narrative damage.
Final thoughts: missing an eye isn't a trouble and it isn't worth compelling; it's a player detail. Unless they're somehow armed with a pistol and fire it off regularly even though they know they're likely to miss whatever they're shooting at. I assume that's not the case, because that would be ridiculous.
As for being curious: what's dramatically interesting about being curious? Not being merely nosy, but the drama of accidentally uncovering something too big that you're not prepared for; of picking up a stray dog by the ears and then being stuck with a new crisis of your own making. So lead your compels in that direction, not toward merely being a noodge at dinner.
Finally, if the wizard screws up technology (and I love that, as I love urban fantasy in general), the explosion should have gone off BEFORE the safe was opened (or whatever they were trying to do): all the noise and commotion, none of the payoff. That leads to an "oh shit" scene ("Everyone heard that! Let's get out of here!") followed by a "What now?" scene ("The police are going to have that entire place cordoned off. Is there another way in?"). Possibly the person closest to the explosion also has Temporary Tinnitus until the end of the session. That feels like just about two steps of difficulty.