To go point by point here:

"What's your philosophy playing Diplomacy?"

I come in to have a good time; as my record shows, I'm no world-class tournament winner. In-game, I'll go with whatever works, with the relative constant that I love fleets far more than armies, and so love to play the fleet-strong nations.

"Does it change depending what country you play as, or do you have an overriding vision?"

Well, first, my feelings on the countries:

England: Tied favorite with Italy
France: Eh...for some reason I just can't do as well with France, odd...
Germany: I tend to either go out quickly or go out slowly...
Italy: Tied favorite with England, and I'll make a claim to being one of the best "tooth and nails" Italys compared to others here insofar as I'll hang on as long as possible even with near-certain death and maybe eke out a survive, a fun nation for me
Austria: Outside of England and Italy my favorite because I'll either go out so fast there's no chance to get upset about it or I'll have a good game, easily the army-heavy nation I prefer and I think I do best with
Turkey: I get handed Turkey, sigh, hope for the best...and I've not often gotten it...
Russia: I'm a pitiful Russia, Mr. Chekov, Mr. Chekhov, and the Pole-Russian half of my ancestors must be ashamed...

So does it make a difference? If I'm playing Austria, maybe, since I don't have my beloved fleets, because the overriding philosophy I have for the game is to eke out a draw, I'm not an Ubermensching soloist, those fleets keep me alive, so if I'm Austria, I need to find another way, because I always try to make sure there's an escape hatch just in case...

"Are all nations created equal Spring 1901, or are some Masters and others Slaves?"

Aristotle or Nietzsche would look at the board, watch typical game and say, "YES, OBVIOUSLY!"

And surprise, surprise, I agree- I'm an ITALY player, of COURSE there are greater and lesser nations! By design, I don't think it's a stretch to say Italy and Austria have the weakest starting points, Italy because of the issue of building post-Tunis and Venice is right next to Trieste, and Austria because she's surrounded on all sides and, oh yes, Venice is right next to Trieste. France has a pretty good starting position, as does Germany in my opinion; the Kaiser's land-locked in between fronts, but generally Russia's so occupied with some combination of England, Austria, and/or Turkey early on that I haven't seen too many games where the Eastern front flares up badly for Germany early on, leaving the West, which IS a battle, but Germany's in a good position for the fight. England and Turkey are interesting cases; as corner powers and natural naval powers with natural oceanic defenses, they have a good starting position in that sense, but trying to pick up centers can prove a challenge if their enemies are determined to shut them out, and so the art of diplomacy is needed just as much as those fleets for them. Russia's extremely hit or miss, either she dies first or second or becomes an absolute behemoth.

However, this does all depend on whether the PLAYERS are Master or Slave types, in the Aristotilean sense of those therms. A Master player will come out believing he is superior, is more likely to solo and can solo and should aim for the solo, and will perhaps use more, shall we say, "mine's bigger" diplomacy (you can decide what "units" he's calling larger there) than a Slave player. A Slave player is a firm believer in the draw, will try and be more diplomatic, and play along the stalemate lines, and while they'll go for the solo if it's there for them, most of the time they're gunning for the draw, and an alliance- they don't mind submitting themselves as much as a Master might.

So how does this affect the gameplay? Well, say you've got a Master mentality in England or Russia, both nations that need to diplome well wearly on, England to get onto the continent and Russia to convince everyone she won't murder them. Russia saying "my forces are bigger, do what I say" in the early going isn't going to win him friends, will either piss of or scare off those friends, actually- and he's gone. On the filp side, say you've a Slave player in France; I'll go on record and say of all the powers I expect France to solo most often (and in second I'd put the escaped-death Russias and Turkey behind him.) France NEEDS to show off some of its muscle, because it's gains are a double-sword; you'll get more gains and builds, but also you'll become a good target early on. This requires a player who's capable of using "forceful" diplomacy, rather than a more passive approach (maybe this is why I do so badly as France, when I come from playing Italy so much...)

And...that's a start, and now everyone knows free info on obiwanobiwan and will beat him round the head with a stick in every game! :p (Though this happens enough already, so I don't know if you really NEED the info to beat me, but what the hey, I'm a casual player except for big tourneys...) :)