In general I loved the added breadth that 3.5 brought to damage reduction. I really value DR #/alignment and DR #/special metal. The only weakness I've found came from collapsing all the remaining damage reduction into either DR #/magic or DR #/epic. These two forms of damage reduction typically offer no protection against the opponents that the creatures with such damage reduction typically face.
I think that the portion of damage reduction represented now by DR #/magic or DR #/epic should be re-expanded to require specific enhancement bonuses to overcome. Here's what I've done as a house rule in my campaign: when a creature has DR #/magic now it gets the DR #/+X it had in 3.0; when a creature has DR #/epic now it gets the greater of DR #/+6 or the DR #/+X it had in 3.0.
I've made a few other minor tweaks. For common templates with DR #/magic, like the half-celestial, I've had the DR start as #/+1 and rise over the course of 20 levels to #/+3. For dragons I've adopted the following rules: Change DR 5/magic to DR 5/+2. Change DR 10/magic to DR 10/+3. Change DR 15/magic to DR 15/+4. Change DR 20/magic to DR 20/+5.
Currently your alpha rules give a 20th-level fighter DR 10/- when using the armor he or she has mastered. While the damage reduction applies only to one specific type of armor, in practice the fighter will virtually always have that type of armor, and therefore virtually always have DR 10/-. Heretofore DR #/- has been, for player characters, the exclusive province of the barbarian.The barbarian at 20th level, however, gets only DR 5/-. At a minimum, then, a fighter should get no better than DR 5/-, and even that infringes on the uniqueness of the barbarian. I would prefer to see the fighter, like a warforged with adamantine body, get DR #/adamantine. For the 20th-level fighter I'd accept DR 10/adamantine, or perhaps even 15/adamantine.
Don't forget that they are revising Barbarian also which will be shown along with all the other classes in the beta release. I suspect that when it comes out the fighter won't be stepping on its toes so much.
-Tarlane
I haven't forgotten, but I doubt that they'll more than double the DR for the barbarian so that at 20th level it grants more than 10/-.
Considering that fighters get no DR until 19th level, at which point they have to choose a specific type of armor or shield to gain DR 10/-, it doesnt overshadow the PHB barbarian much, IMO. Starting the barbarian's DR at 4th level would probably be about comparable (up to DR 6/- at 19th), since barbarians get the benefit for more of their career and gain that benefit regardless of what armor or shield they use.
Considering that fighters get no DR until 19th level, at which point they have to choose a specific type of armor or shield to gain DR 10/-, it doesnt overshadow the PHB barbarian much, IMO. Starting the barbarian's DR at 4th level would probably be about comparable (up to DR 6/- at 19th), since barbarians get the benefit for more of their career and gain that benefit regardless of what armor or shield they use.
As I indicated, the figher will virtually always have his specific type of armor, so that limiting it to one type of armor in effect limits it barely at all. DR #/- has been the exclusive preserve of the barbarian, and it should remain so. Under your argument you might as well give fighters Divine Rage at 20th level because that wouldn't be too much better than what barbarians get. I have no problem with improving fighters a bit, but not at the expense of barbarians. Give them more of the sort of thing they already get, like more fighter feats, or special abilities that deal with specializing in weapons or armor--but create something different than what barbarians get.