Telika: Well, you forgot the distinction between being able to pick it back up immediately, versus being able to leave the area, come back much later, and pick it up. While there is something to say for the realism of people (or other creatures) eventually picking up what you dropped, the fact that they vanish after a while is often just an engine shortcoming.
RPGs, with their item-swallowing grounds and buggy containers, made me paranoid about dropping and even storing stuff away. I'm never at ease with it, no matter how solid the game (meaning that past experiences sometimes ruin my enjoyment of new games). But item persistence is, in my eyes, a quality factor. It's the first thing I check in FAQs and guides.
If I remember well, Morrowind was relatively good about it. Project Zomboid too. Maybe Fallout3 as well ? Thing is, my memories are blurry, given that even when playing, I tend to shy away from testing item persistence (or just go "okay, it stayed there so far, but maybe NEXT TIME it won't be there).
I'd also say that a difference between adventure games and RPGs is that adventure games with limited inventory space suck.
That distinction doesn't really seem to separate genres. If we compare, say, Ultima 6 and Wizardry 8, those games still feel like they are in the same genre. (While we're at it, we could add TES: Arena to the mix.)
In any case, these games are all WRPGs that pass the test, but they vary in terms of how they handle the case of dropping an item and leaving the area. For these examples (and throwing in Dragon Quest 3, a JRPG, as a point of comparison):
Ultima 6: The item can be picked up if you don't leave the area. If you do leave the area, some items (which I refer to as "transient") will disappear; these usually include items that are created during the game (like those you purchase or that enemies drop, for example). Other items (including those that exist from the start of the game, and all quest items) are what I call "permanent"; they do not disappear when you leave the area. In this game, you can preserve transient items by putting them into a permanent container (the chest in your room in Lord British's castle works here). In the reverse case, if you put a permanent item into a transient container, when you return the item will be on the floor. Quest items can be dropped, and because they are permanent, they will be there when you return.
Wizardry 8: Items can always be picked back up. There is actually an issue with this: If you put too many items on the ground in an area, the game will slow down in that area, and there is no good way to delete many of these items. Quest items can't be dropped, but you *can* put them into containers; doing so with the artifacts can cause strange effects (including making the game unwinnable if you leave them in the Rapax Away Camp).
TES: Arena: The item can be picked up immediately, but if you leave the area (or even just change floors), the item disappears. Incidentally, Daggerfall is like that, except that items stored in a house or ship that you own do not disappear.
Dragon Quest 3: Dropping an item will cause it to disappear for good; there is no picking the item back up. (This game fails the test, as you can do 1 but not 2.)