Non destructible environment.

I'm a walking god of war in power armor and a nuke thrower, but I can't knock down a freaking wooden fence.

Try "every game's biggest weakness". Very few games actually get away with truly destructible environments.

There was a mod for New Vegas that attempted this, and ended up serving as a really good example of the problems designers would need to overcome. Pathfinding becomes a nightmare, saves bloat up incredibly with data about what's destroyed and where all the pieces are, and that's not even getting into the level design; there's not much point in Bethesda designing a tricky scenario or an environmental puzzle if everyone can just blast a beeline through their locations.

well i wouldnt say its the greatest weakness....maybe a lack of detail in certain areas (with a lot of detail in others)

you do have stuff flying away when blowing something up, but not the whole structure colapsing - i feel this is a good compromise... if buildings collapsed all the time, this would create a huge set of various problems...(not finding stuff/people anymore, having to constantly repair buildings in settlements etc)

i hate games with destructable things

Can you imagine how easy it would be navigating Fallout 3's downtown if destructible environments existed?

That would depend on how things were implemented.

If you could destroy some things easily but if massive changes required serious work, if you had to also worry about things collapsing on top of you? Well... I guess maybe there's a reason that in real life people do not drill tunnels with pistols?

I am not saying that design and implementation would be easy, just that destructible environment by itself needn't be a game killer. (But games will always have limitations...)

If you can knock holes in walls you lose all game design.

IMHO it worked for the first Red Faction quite well. However you could not destroy everything, and explosives were not that abundant. So if game design factor in an destructible environment it could work. But actually I won't suggest this for Fallout. Bethesda should make those features that are already in the game work better - that will be work enough. And IMHO the biggest weakness is quest design.