A planewalker's definition of TN could encompass everything talked about so far. The Outlands' native rilmani (as well as the petitioners) are the obnoxious "keep-the-balance" type, to the extent of switching sides in the middle of a fight, if need be. Sigil itself is the epitome of TN as the ultimate mixing-pot (the whole "deva drinking with a fiend" illustration).
However, I have found the simplest motto for most TN folk is, "Look out for Number One". Animals are the perfect example. When they kill, it is for simple survival. When they nurture their young, it is survival. It is not good, it is not evil, it simply
is. They don't go out of their way to hurt anyone, but they don't make an effort to help them, either. They live for themselves, not out of selfishness, but as a matter of survival.
I never understood why they made druids fight for "the balance". Druids shouldn't give a fig about moral or ethical balance. They have no stake in it--that's the clerics' turf. Ecosystem balance, yes. The other stuff is for civilized folk.
Accordingly, it should be noted that there are many shades of other alignments, too. The oft-maligned Lawful Good is not just for the Superman boy-scout--it's for Batman, too (doubt it? Then why doesn't he just kill the Joker and be done with it instead of repeatedly tossing him in Arkham?). Chaotic Evil is a tanar'ri, yes, but it's also the Bridge District Skinner (who, in my opinion, is ten times worse than any fiend you face in
BG2, in terms of evildoing).
Anyway, just my two coppers.