Not worried about graphics? Poor voice acting not a problem? Decent animations 'alright for these young kids, they like that sort of thing'? Prefer loads of items, ingredients, spells, customisation and options? Some politics thrown in? Variety of monsters? Bored with dungeon crawling, let's have most quests above ground, please? Like choices in quests or the ability to fail? Tired of hand holding, want to be butchered mercilessly if you enter the wrong area unprepared? Epic plot involving betraying or protecting the world from usurper gods, with apotheosis the reward for betrayal? Ladies and gentlemen, the original Two Worlds had it's good points too.

I'd say that New Vegas managed to pull off the in-depth quests and NPC interactions pretty well (and things like different ammo types helped spice up combat a bit), although their free-roam world was a bit more dull to explore than in a Bethesda game. My dream game is basically to have Bethesda handle the world-building, Obsidian handle the quests and characters, and either From Software (Dark Souls) or 38 Studios (Kingdoms of Amalur) the combat.

38 Studios won't be designing any new combat systems for a while...

And I've said it once and I'll say it again: Obsidian doesn't need Bethesda's help in designing open and explorable worlds; Honest Hearts and Old World Blues prove it.

My opinion only : Old World Blues proved to me that Obsidian can sometimes sacrifice your ability to actually enjoy playing a game for giving you story, background and npc conversations (and they got it so right, again imo, with the vanilla game).

Personally I just don't find their worlds as enjoyable to explore as Bethesda's, I'm not quite sure what it is, I think it might be because while Obsidian are good at writing actual written stories, Bethesda are good at telling small ones using props and the environment itself, which makes exploration much more compelling and rewarding to me. To me, Bethesda worlds feel more organically interesting, kind of like exploring a new city (you've got the major sights pointed out to you, but along the way you can find all sorts of cool little things), while New Vegas was more like an art gallery, where you've got everything laid out, and in between the works themselves you've just got blank wall.