The way I define things, an action or adventure game is all about the action and there is no support in the game for roleplaying. An action RPG has a focus on the action and a limited ability to create your character, and an RPG has full character creation and a reasonably large amount of focus on the roleplaying as well as the action, tyically manifesting in such things as dialogue options and choices.
Like all such definitions, there can be exceptions and games that blur the lines. It's very difficult to define what makes a roleplaying game, because many people seem to have quite different definitions of what it means to roleplay. Character creation and control is very important to me, but some only want one or the other.
Some other things that are important to me in a face to face game are by necessity limited in a computer game. You can't have the NPCs really reacting to what your character does in a computer game -- it would be insane to program in a response for every possible thing, and so we end up with dialogue options. When it gets to the point of extremely limited dialogue options (most of the time having only one or possibly two) or not being able to imagine how the character is saying it, then I start to think of the game as less of an RPG.
Choices and consequences and changing the state of the world are all bonuses to me, but so long as it doesn't seem like the character should very much have been able to effect the thing in question but couldn't it doesn't bother me if it isn't there.
Combat is also something that I'm quite fond of and like to have in RPGs, but I don't consider it a part of the definition. I suppose basically I define whether something is an RPG or not by the amount of character creation and roleplaying potential there is, and then if that is lacking, define it by whatever else remains primarily.
I consider the Infinity Engine games to be some of the best examples of RPGs there are, because for me, they allow great freedom in character creation and control. There are enough diaogue options usually that I can find one that fits, I can make any characters I choose to, and the only real limits are the limits of 2nd edition AD&D (I could well do without the racial class restrictions, as I always did in face to face games, for instance). There is nothing as a particular obstacle to roleplaying. And, while I in many ways prefer the Baldur's Gate series, the Icewind Dale games come up higher on the roleplaying scale for me because there isn't even the hurtle of a partially defined background. I'm also of the opinion that combat that depends on the characters statistics and such is more about roleplaying than combat where you control everything they do, because then the outcome is based off of your characters abilities and not your own, and they certainly meet that requirement.
I think this is how many people think about these types of games, but I have to ask: Why is an "Action RPG" like The Witcher 2 not a "real" role-playing game? The action isn't actually the point (Baldur's Gate certainly has lots of action), the difference is that it's realtime. And this doesn't actually diminish any other features associated with the RPG-ness of a game (the game has choices, consequences, different ways to level your character etc.). So why is a "realtime RPG" not as much of an RPG as a "turnbased RPG"?
I have not yet got around to playing The Witcher 2, but for several of the others on the list I would say the problem lies in not enough freedom of character creation or of roleplaying choice. I don't know how much you can choose about your character in The Witcher. I'm guessing at very best it would fall into the same category for me as Planescape: Torment does (as in, enough sheer choice in how you play the character that the fact that you can't create the character to begin with doesn't keep me from thinking of it as a roleplaying ame), and at worst it would be severely limited and so I'd categorise it as an action RPG.
This is what I don't understand. For me, games like Baldur's Gate are actually some sort of hybrid between RPG and strategy game. In combat, you do not roleplay a certain character at all, you simply play up to 6 different characters. (Of course nobody forces you to control these other characters, or you could go solo, but I believe when we talk about BG2 we think of tactical party-based combat.)
You do roleplay a certain character or characters. Roleplaying more than one character at a time doesn't make it less of roleplaying. Granted, bringing an entire party of NPCs makes it a little shakier, but you still very much create and control the one PC you have then.
Arcanum, Fallout, Mass Effect, The Elder Scrolls and The Witcher let you control exactly one character. One could make a good case that they are the real RPGs.
I'd say that Arcanum, Fallout, and the Elder Scrolls games are all real RPGs. I haven't played The Witcher yet, so I can't say for sure my opinion on it. Mass Effect skirts the line to me, because there is that dialogue wheel strongly limiting the ability to roleplay the character. As the series progressed, it took a nosedive into not being much of an RPG at all.