armors dont directly reduce your movement.
just use table 9-2 "Carrying Loads" in the PHB, page 162
remove the "max dex" column, and replace it with:
Medium: -2 dex (more?)
Heavy: -6 dex (more?)
this dex penalty doesnt effect skill checks (the armor check penalty already does)
(emphasis mine)
Why have an armor check penalty, then? The Dex penalty affectively does the same thing, doesn't it? If medium armor reduces effective dexterity by -2, then you have innately imposed an armor check penalty of -1 to all dex based skills. Perhaps some medium armors are more restrictive of movement. They might shield more while applying an additional -2 dex penalty (for a total AP of -2). I actually rather like this more than using two systems to deal with the potential negative affects of armor.
One more point to consider: Str based skills are also affected by armor penalty. Granted, the Str bonus counts against this (as suggested in another post, and touched on below). As for Swim - I think the armor penalty also applying to it (once) is enough - especially if you add an additional negative for each degree of encumberment (due to weight). Perhaps light encumberment adds an additional -2, medium -5, and heavy -10?
Hmm, I plan to use an 'armor as DR' varient soon - perhaps permanently, so I'll have to think over this a bit more as the balance will notably be different in such an instance with such a system.
Discouraging archers from heavier armors isn't a bad thing, but encouraging other high-Dex characters (like rogues) to wear heavy armor isn't good, IMO. I guess it all depends on your tolerance for these sorts of things and your relative preferences.
Recall that for a rogue to wear heavy armor effectively he must use a couple feats to gain proficincy. If a PC is willing to blow two feats just to wear heavy armor as a rogue than I don't mind. The penalty to move silently will still get him when he attempts to be stealthful - such as to sneak attack. It will also get him when he attempts to evade traps.
True, the rogue can multiclass to also gain that benefit, but their are other advantages (and disadvantages) with that approach, so it evens out somewhat. A Ftr 2 / Rog 8, for instance, could make use of his ability to wear heavier armor (with the penalties mentioned above), but he is out 12 skill pts, his first rogue special, etc. So either way - feats or multiclassing - the rogue has to pay to make use of heavier armors. This is enough discouragement in my opinion.
What if armor had an "encumbrance" rating which gave it dex penalties, but the penalties were partially reduced by high strength.
Let me see if I understand this correctly.
A character with Str 14, Dex 14 puts on medium armor (above suggested for Dex -2). Their AC is now 13 (armor +2, dex +1). His dex bonus to dex related skills is now only +1, and due to his strength he is not encumbered.
On the other hand, a character with Str 14, Dex 14 puts on a set of full plate (heavy armor, above suggested for dex -6). His AC is now 17 (armor +8, dex -1). His dex bonus to dex related skills is now -1, and due to his strength (+2, vs the -3 to dex) he is lightly(?) encumbered (in terms of movement, not necessarily in terms of weight).
I'm not sure if this works or not. I think we are on to something both interesting and useful, but we need to work on it a bit more. Recall, however, that the penalty to movement is due to the fact that the armor restricts the movement of the joints. It not only slows the movement in general (due to weight), it also limits how far each joint covered by the armor can bend - thus limiting stride, etc. Recall also that half plate, chain shirt, etc are presumed to include other components that - collectively - cover most of the body.
