I think I'm confused now.  If I'm Taken Out, can't my opponent slap me with a consequence of their choosing as part of the "my ass is theirs" clause of Taken Out?

Being Taken Out should NEVER be a good thing, at least that's how I see it.  Else the concession rules wouldn't need to be there.

I think you are looking at consequences through the wrong end of the scope.  Consequences aren't how you get beat up.  Consequences are how you justify how your character keeps going when they should have been stopped.  They are used to not be taken out.  Of course, there is nothing stopping you from placing an aspect on a taken out character if you feel like its necessary to give that result force.  

Edit:  Look at it this way:  Consequences provide a choice between not losing a fight at a cost, or losing the fight.  If you still inflict consequences when they lose the fight, then there is no reason to hold back on taking consequences.

The GM's responsibility is to tell a story, and to try to ensure everyone at the table enjoys it. Since different people have a different view of what is fun there's no way to say that a GM should always provide this or that thing. If you'd read the context, and the situation clearly you'd see that both of these responsibilities seem to have been upheld.

Besides that as a GM I have been in so many situations that would have been a conflict of interest (as you describe above) if I was not aware that my only interest was the table's. Realizing that you're not opposed to the player, but working with them is what makes a great GM.

It's not the GM's responsibilty to tell a story.  Maybe the game can be played that way, I don't know, but its not the automatic default.  The context is that the GM and the player disagreed about what should happen, and about what would be more fun.  This shows that there is some sort of breakdown going on somewhere in the game.  This is not a sign that everything that happened was perfect for his groups social context.

You misunderstand what I mean by conflict of interest.  It's in the GM's best interest to make the conflict with the NPC something that poses a genuine threat the the PC(s).  This responsibility is spelled out in the rules.  If the GM is also given the responsibility to manage credible opposition against the NPC, then he is forced to choose whether he will manage that opposition credibly, thereby undercutting its ability to oppose a PC, or whether he will advocate for the NPC being a threat, thereby undercutting the credibility of his opposition to the NPC.  Characters cannot gain +24 shift attacks without having the opposition to those maneouvers and declarations being managed by the GM.  Thus, if the GM pretends to treat the NPCs the same as the PC's, he has introduced a clear conflict of interest, in which he must abdicate one of the responsibilities he has taken upon himself.  If he wants to create an unwinable fight (for the "story"), he can do so without such a conflict of interest by treating the situation as a pure compel, or even by just saying "your character falls in love, no recourse" if the group will put up with it.  But the GM cannot credibly approach the situation in the way the OP described without messing something up.

First of all, a 24 shift attack from a pure mortal (or anything less than a faerie queen, really) is pure bullshit.  Yes, I know you found cute ways to justify it, but you still pulled the equivalent of "you lose because I said so and I have all the power". 

On the main issue, yes, the player decides if they take a consequence or not.  The downside being that when you lose a conflict, you are at the whim of whoever beat you.  This is one of the big strengths of the Fate system (the aspect thing overshadows this, which is a shame).  Because the player can determine when they take consequences, and whether or not they concede or press the issue, they have the power to determine what is important to their character.  They can say "I'd rather lose this fight here, and save my strength for something else" or "no, this matters, I'm pulling out all the stops, regardless of what it costs me in future effectiveness".

There'd be roleplay-focused, I suppose, where one feels is IS okay to roleplay for RP's sake, to meander through the PbP with character interactions and introspections and puppeted encounters with NPCs.

Notice that this is definitely NOT what Mij is saying.  Even in his roleplay-focused group (which I have spied on once or twice), they do not spend a lot of time (or much at all, really) on navel-gazing, nothing goes anywhere type of "roleplaying".  Every scene matters, its just not necessarily a mechanics intensive form of mattering.

I'm pretty sure Seoul ended when Bosh disappeared without warning.  I have no idea why that happened.

Sadly, I've already forgotten whether I called an end to Seattle (Hard Rain Falls), or just left it hanging.  I do recall the circumstances which occurred around the time it died.  One player had dropped of the face of the earth, and wasn't responding to messages or e-mails, and I was on a lengthy trip without consistent internet access.  The game went on hiatus to find a replacement player, but after some time, the momentum for posting was clean gone.  It was shortly after that that things ended.  I was getting burned out at the time this all happened.

The GM is probably a huge factor in how long these games can last. If your Pbp game relies on a GM, you should have:
a) countermeasures in place in case the GM stops.
b) A good posting rhythm from the players.

Of course, the real key is having a bunch of players who are enthusiastic about telling the stories of their characters together.  But all the other advice can help a lot.

... an other thing to consider is, that concessions aren't NPC only. A beaten PC may also concede to save his hide and proceed on an alternative path. Usually I don't like to draw on the novels too much during rule discussions, but Harry does this all the time! If the game table doesn't allow the reasonable concession of a NPC then - in my opinion - the hole concept has to be taken into question and a cornered PC shouldn't get to concede either...

This is a really good point.  By the nature of concessions, what is acceptable will vary by group.  But the kind of concessions the players insist on for NPC's will likely reflect on what kind of concessions are acceptable for the PC's themselves. 

If the players are uncomfortable with how concessions are being used, I'd first try to host a group discussion about the purpose and acceptable range of concessions.  If the players were still uncomfortable with how I was using them, it might just be better to shrug and let the players preferences determine in the abstract how concessions are to be treated.

I'd say that true love is never temporary, but then again I'm a romantic that way.

Well, whatever form of love actually effects WCV's is temporary, as shown by the fact that Harry lost his protection.

The correct response is to say yes.  Seriously, its one of the rules.  Several of them in fact, for that particular example.  The person who takes someone out always decides what happens to the person taken out (as long as its reasonable, and killing from feeding happens enough in the books that this example is very reasonable). 

Killing someone with incite emotion gives emotional vampires an immediate free recovery.  This is also one of the rules.

Saying no is flat out, no gray area, cheating.