Damage Reduction

I've noticed a trend in games toward the use of % based damage reduction over the simpler flat number based damage reduction common in old games. While this is a good thing for balance, it tends to make things less interesting.

Flat number based damage reduction varies in it's effectiveness depending upon what it is up against. This can make balancing rather difficult, as some attacks can become useless against it, while others are hardly affected at all. (In Fallout 2, the mini-gun was a poor weapon choice because against enemies with armor, it'd do almost no damage, and against enemies without armor, it was overkill.)

% based damage reduction is equally effective against all things, making easier to balance. However, that also reduces variety in game experience. A part of what makes games interesting, is the variation in what things work best for a given situation. If damage reduction is equally effective in all situations, it just becomes boring. (and far less interesting when presented as a choice)