Potions (and other similar items, like scrolls and wands) can be roughly divided into two types: permanent and temporary ones. Permanent ones have effects which last for the rest of the game, and temporary ones help in one battle (or another short amount of time). This division is not very exact, it is rather intuitive (if you use a temporary potion to avoid death, then it has a permanent effect, in some sense). But it is mostly temporary potions which cause problems.
Let's see how potions work in several games.
- Diablo has its health/mana potions which you have an infinite supply of (from the shops). You can win difficult fights by drinking lots of these potions. It seems that most people who consider this think that this is mindless and badly designed.
- Dungeon Crawl has several important permanent potions (cure mutation), and several powerful temporary items (potions of heal wounds, powerful offensive wands). These temporary items are important; I think that most Crawl players run into situations where they need to use a temporary power, but the difference between winners and losers is that for winners the use rate is simply lower than the finding rate. On the other hand, weak wands and potions are mostly useless.
- In ADOM, there are several important permanent potions, and lots of temporary potions and wands. I use the spenseweed for healing (since there is an unlimited supply of it), but other items are simply collected and never used. The only wands I use in ADOM are wishing and cold, for their permanent effects (cold is used for freezing ice).
- Dungeons of Dredmor seems to have some potions and mushrooms which have weak temporary effects. Since the inventory is limited, it is not even worth it to carry them. But even if the inventory was not limited, they would probably end up just like in ADOM.
- Hydra Slayer has four permanent potions and scrolls (Power Juice, Life, Big Stick, Reforging) and several runes/potions/scrolls with temporary effect. I am quite happy with all of them, but the reason is that in some sense, everything is permanent in Hydra Slayer. That is caused by the fact that health does not regenerate (and thus it is a permanent). Even though the visible effect of, say, Potion of Ambidexterity is temporary, it also has a permanent effect of saving you several HP.
- I have heard that ToME gets rid of temporary items in general. A good move, I think.
The other thing that bugged me about liquids in VoI is that they are supposed to be not discrete: you can drink half of the potion and get a partial effect. This makes identification work strangely (you can identify potions by just tasting them, so why the need to identify them at all?) Also you are able to create liquids magically via Rune of Creation. But I find it very hard to explain why you can create, say, poison, but only use it immediately, and not fill a bottle with it.
The planned design is as follows. Temporary potions and herbs expire, so you can no longer carry them in your backpack for a long time. If you find a herb, you can either use it immediately (for a powerful effect) or ignore it. Alchemist huts will have cauldrons which magically generate powerful temporary potions; you can drink them, coat your weapons with them, or fill your bottles (but they will stay there for a very short time only). There will be also a portable version: wands which generate liquids with temporary effects when zapped or rubbed. As with other VoI wands, they will need a contact to a living body to work, so you can use only a limited number of them in a given time.