The analogy to gold is a good one. It never makes sense not to pick gold up, and as long as items can be sold for gold, it never makes sense to not pick up items either.
The analogy to gold is not a good one. Gold takes no inventory space whatsoever, and it's fine to hoover this up. There's no 'special' gold or anything like that. If you auto-pick up all these items, there's a TON of problems with this. Not the least of which is the loss of seeing that yellow or gold name pop up on the ground. This is an exciting moment, and it's part of Diablo. Making everything get auto-picked just...removes the excitement of the drop.
Furthermore, your inventory is not going to be THAT huge. And TPs are out, remember. You're not going to be able to just fill up your inventory full of gray/white crap and just TP, sell, come back right where you were ad infinitum. Personally, I only ever picked up magic items in DIablo 2 and near the end game I stopped picking up magic items that wouldn't be of use to me even if they were decent.
Even if you're going to say "Oh I'm going to just pick up all this gray/white stuff for salvage", even then, the gray/white stuff's salvage is not going to be very worthwhile for much of the game. The item level of gray/white stuff tends to be low, which means the salvage level will be low, and it will only give very basic classes of material, if I'm interpreting what we've seen right. It's just not worth it. If you wanna have a stash full of 'Pig Iron Scraps' at level 50, when the highest level item that goes into is meant for Level 8s, then more power to you, but I'm not going to be picking up everything for salvage, and I don't think most people will either.