Though I appreciate that you took the time to write your opinion on the matter, I too feel like I should maybe make something clear: I don't care about this stuff at all!!!
The reason is quite simple: as you suggest, some personalities click naturally, some don't. Some have longstanding histories with players on this site, some don't. Though we like to believe that each game is fresh and new, that we all approach the game with a kind of 'tabula rasa' attitude, we don't. There's context. Always. In every game.
Like ours for example...
I know what to expect from you MM, in some ways at least, but not from most of the other players. And we do have our history, which just might be prompted into a situation at some point, framing our negotiations so to speak. Then, this thread gives me some insight on the ways in which Julien thinks, I can guess at some of his values, and that's going to be 'evaluated' by me whether or not I'm aware of it. The same goes for Gen. Lee, etc.
Where Sly and I are concerned, we've never ever hid our friendship from other players, we have sought out to organize games with other friends of ours, we have taken steps to avoid such issues. The fact is, however, that just like I know some of his idiosyncratic traits, I know some of Lando's, some of Tom's, some of JMO's... some of many players I've played with here and have enjoyed enough to pay attention to who they were. And in all of those cases, that information was useful in games. Did I capitalize on some of Sly's traits? Hells yes. Did I do the same with other players? Absolutely. Do most good players work in that way, whether consciously or not? I'm wiling to bet my right to play on this site that they do!!! And that includes you, dear MM!!! But either all of this is metagaming, or none of it is. Given that I think the game impossible to play otherwise, that is, by abstracting from such info, I want to say that none of it is, that it is a natural facet of the game. Diplomacy, after all, is never far from psychology...
So yeah, that's how I say that I don't care about this sort of issue. I care where there is the conscious decision to obfuscate the truth, manipulate others, and willingly cheat towards victory. To me, that's the only kind of metagaming that is wrong. But where it's a question of what you know about other people, and how this factors into your choices, that's just the nature of the game.
And I will finish by adding, MM, that there's at least one game where I can show beyond the trace of a doubt that you've capitalized on what you had understood of Sly's personality, that very same trait that has made me work with him on so many occasions. I have good reason to believe that it's information you've also used in other games you've played with him. I'm not going to flat out say what that is, of course, but I think you'll know what I'm talking about.
That's my 'two-cents' on at least part of the "meta-gaming" issue: there is always something beyond the game that frames the way we approach it and play it. Games are not played in a vat. And what we know of the players we play is significantly part of that frame. Which is why we have anon games. Which is also why we deliberately try to have "pure anon games". In both cases, the aim is to reduce the impact of that knowledge. But that's to say that is has an impact in most, if not all, cases.
On the topic of "pure anon games": I've played one with Sly, that is, I've played one where he and I were both in the game without knowing that the other was in it. I even made an effort not to care about the way people wrote: had little chance of guessing who was in it, given the set-up, and thought it a waste of time (and, also, contrary to the spirit of the game). Well? I still allied with the fucker!!! Something in our exchanges made me think that whoever I was talking to had that "trait" that I could work with, that trait I've said Sly has. And so we worked together. I never had a clue that it was him until the end.