In the Obsidian and Bethesda forums, J.E. Sawyer talks in more detail about the removal of Damage Resistance in favor of Damage Threshold.

Fewer variables are easier to tune than more variables. I tried to do everything through DT and I think the results are pretty good. I left in DR in case my math on paper didn't translate to fun in the game. It also allows PC modders to run buck wild with it if they want to. The GECK/engine/display still support both.
Given how armor works in practice in F1/F2, I am extremely skeptical that the DR/DT system was designed to "combat" picking the armor with the highest DR/DT. A system is nothing without content, and the content of F1 and F2 featured a form of power armor as the end-all-be-all endgame armor. I mean, you could finish F1 in standard combat armor or Brotherhood armor, but power armor/APA is so much better that it's mostly an aesthetic choice. Armor does follow linear DT progressions upward, but outfits are now divided into Light/Medium/Heavy classes. Light moves without any movement penalty, Medium with some, and Heavy with more. Of course, some armor types come with their own built-in bonuses and penalties. I think there are compelling reasons to use different armor types, even toward the high end.
No creatures have DR. Bullet sponge enemies have a lot of HP and typically don't have DT. High HP characters with no armor are begging the player to use low DAM, high DPS weapons on them. In my current playthrough, I definitely make specific tactical choices about when to use certain weapons/ammo based on enemy armor/health/range. The anti-materiel rifle has a very low DPS for when you get it in the game. There are some weapons you can get very early in the game that easily eclipse its DPS by a healthy margin. But the sheer amount of damage it does in a single shot can bypass pretty much any DT, making its effective DPS much higher against certain targets.

Power armor is self-powered, but that does not mean that a) the power under which it operates completely offsets the weight of the armor b) the power is fixed, as opposed to a force multiplier c) that the power of the armor necessarily offsets any articulation problems inherent to the design.

But more importantly, statistics are given to equipment to accomplish a game design goal. Things that I think are problematic about what you are proposing:

  • Establishing a fixed STR in power armor means that the player's STR stat is effectively irrelevant once PA is found.
  • Establishing a max AGL in power armor means that player AGL above that stat is similarly irrelevant once the PA is found.
  • Power armor being weightless would, on its own, be a pretty practical reason to use it.
  • If that were not enough, power armor has the highest defensive statistics.
It winds up simply becoming "the armor you wear in the last X hours of the game" as opposed to one of a few choices.

He also corrects a mistake on my part from my New Vegas preview:

I should have said this earlier, but weapons do fixed amounts of DAM, not variable. DAM is based on the GECK setting, player skill, weapon condition, ammo type (if applicable) and other effects like chems (e.g. Psycho). There is no die-rolling for damage (outside of critical hit chance).

Update - two more posts:

[Gizmo: This is true... This is also basically how it was in Fallout 1 & 2 though... and I don't really see the reason against it.]

Because it removes decision-making from high end armor. "Would you like to wear power armor or some other armor that is worse in every practical way?" is basically the question asked in F1/F2.

[Gizmo: If a person were alive in a post apocalyptic wasteland, and had the choice between leather armor, metal plating, high-end combat armor ~and a powered exo-suit with filtered air.... Which one would most choose to wear most [if not all] of the time?]

Again, that's not the point. If I, as a designer, have the ability to balance high end armor so a player has pros and cons for wearing one suit or another, why would I not do so?
Nowhere have I stated that leather armor should be a viable alternative to power armor, only that there should be viable alternatives to power armor. The varmint rifle is not competitive with the anti-materiel rifle, but the minigun and the anti-materiel rifle are both very powerful late-game guns with different applications/pros and cons.