proficiency
should be required to maintain dex bonus. wearing plate should be possible, untrained, ala 4e. Maybe you get a to-hit penalty like 4e, too. That would be very tricky for a player to decide if they want +2 ac for -2 to hit...not at all an obvious choice in general (though I can see a rogue putting on plate to fool other guards, or to run through a poisoned bamboo trapped corridor)
I love all this info about plate!! I do think cost in gp should be balanceable, because it means in the first few levels, a fighter just can't afford it. If he find it on an enemy, it probably won't fit, unless he's extremely lucky. Or if you loot an ancient knight's tomb for his armor...
I'd say a balancing factor should be that it takes a month to make a tailored suit for a knight, and the 1500gp should not be given out lightly by DMs.
The ONLY way a rogue should have a higher AC than a fighter is if the rogue is much higher level and has taken all sorts of defensive gear such as Tomes of Dex or whatever. A standard Dex score range should not be equivalent to something most peasants could never even dream to afford.
I have a friend who makes plate armor, and even though I earn a ton of money, I need a layaway plan just to buy it. We're talking tens of thousands of real dollars, all told. It's 2k just for a helmet. Cost SHOULD be a balancing factor...just balance the treasure you find.
In in DDN the proportion of your time spent in-combat vs out of combat is way less apparently than in 4e or 3e, because things run faster generally, so that means the out of combat drawbacks of plate suddenly become huge.
Exploration is definitely hindered with plate on, over the long haul. I'd rather a fighter learn to overcome the speed penalty of plate -- and dex limits, if any -- with levels, similar to Pathfinder. It gives you a good reason to keep your dex up, b/c a fighter should be good with a bow too, IMO. Or at least throwing a handaxe or a spear