One funny thing about the orbs in D3 (or other games that use that system) is, that if you look at it, it’s practically the same as if the monsters would deal less damage:
You just give hp’s to the player for free that he has lost a second ago.
So, why not just reduce the dps from enemy mobs and get rid of the orb-system 
Well, for one, the orbs force the player to move in the area (to pick them up when needed). This makes gameplay more dynamic.
Secondly, as long as the orbs remain on the floor without going poof after a few seconds, there is a tactical element of timing when to walk over them. The player has to decide whether to break a fight to get health or to try to finish the monster off first. This can be an interesting decision.
Thirdly, if you link the drop-rate of the orbs to the kill-death/ratio of the player, you can implement a pretty subtle way of balancing the game for the player. Weak or badly equipped players can still beat the game, they just need to walk over more orbs. The player can tell if he’s performing well when there are almost no orbs dropping at all.
BUT, i still think, it’s silly. The monsters should not give the player the hp’s back that they just took from him. At least not as simple as that.
It would be different, if there would be some sort of skill that the player needs to activate in order to make orbs drop (much like vampirism) or if they only drop with crits (making character-development having to choose between high damage, crit chance (+ orbs) or better defense for example)
Potions, on the other hand, have the issue, as described above, that they tend to neglect the difficulty of any monster: any increase in dps from mobs is countered in gulps of red potions. Thus making the game “harder” is really not as easy as it sounds.
And if you then, in this scenario, restrict the potions, or make the more expensive, then the player feels nothing but frustration.
The only way out, in my opinion, is to give the player various tactics to avoid as much damage as possible. Basically removing the need for any orbs, potions etc. (GuildWars for example does that and if works very very well, alas only for groups with a dedicated healer, another example from an mmo is The Chronicles of Spellborn, where playerskill and various class-based abilities made damage-reduction one of the most exciting part of the combat (the game didn’t have a “healer”-class)). Now, in an arpg you cannot avoid damage, at least not totally. So, how can you reapply it without potions?
Each class should have certain skills that give the player back some hp’s. This can be either over time via regeneration or instantly. Or via vampirism (hp-gain through successfull attacks) or successfull blocking. Or through indirect effects like skills that “flip” a healer from the enemy to heal you instead of his fellows… tactical stuff like that. There are plenty of ways to give the player hp’s through active skills.
Like this you can give the player a choice to rely more on quick killing rather than prolonged killing + better self-healing. And you can much better make the game harder by increasing the dps (or reducing the abilities for self-heal).
So, bottom line: i vote for no potions at all! 