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In the past decade, I have found it increasingly difficult to focus my attention on a single-player game. Occasionally I am able, but it's easier if I'm at least playing with a friend, both of us working on the puzzles.
I still wonder exactly how Quest for Glory V would have worked. There could only be one King at the end, right?
I'm was disappointed in how Neverwinter Nights worked. I could never grasp the interface - I fought against the trendy "Radial" controls. Still, I think a basic concept of NWN - that multiple people can cooperate together on an adventure campaign - would be good to see, in games with a less convoluted GUI.
Massively multiplayer games such as WoW also have adventure elements, but still have a ways to go before they're as satisfying as single player games, because the real sense of accomplishment is somewhat defeated for me when I defeat an arch enemy and he simply respawns 5 minutes later. Knowing that I haven't really changed the game world bothers me... and I cannot help that feeling despite understanding the rationale for how the game works, and that developers cannot churn out infinite content at an infinite pace. Content has to be replayable so many players can live the same experience... even if it makes a victory feel a bit hollow.
That's why I feel the real future for adventure games has to be in the NWN model. The same model could have worked as far back as The Bard's Tale, early classic dungeon delving games where a single player controlled up to 5 or 6 characters. The key is for the other characters to be playable by other people. This has to work for the right size party of individuals... one of the draws in WoW is that Jimmy knows Roger knows Betty knows Susan knows Charlie who plays the game... and that is unfortunately how people 8 or 12 times removed from an "early adopter" of the game as a place to recreate end up on PvP servers.

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I never got the hang of playing with multiple parties in RPGs. If you like that kind of game, you should pick up the old Baldur's Gate series and also try the Temple of Elemental Evil. I started both of these at one time, but lost interest in trying to maintain an adventuring party. Gamespot has a movie on the Temple game and it's still available on Ebay for very little money:
http://www.gamespot.com/pc/rpg/greyhawkthetempleofee/index.htmlI loved the story behind the Neverwinter nights series, but there is no freedom in the gameplay. You follow the story line and there's no re-spawning monsters to continue to boost your levels. Even the old QFG let you park outside of the goblin mound and the minor monsters re-spawned.
The Diablo series allows you to continue to play each level continually re-spawning the minor monsters, and get the maximum amount of points as long as you don't kill the main boss of each level. It also allows you to replay the whole game series boosting the challenge-level up each time. New monsters, and weapons are introduced at each rise in the level of difficulty.
Marten, didn't I remember you trying to return your QFGV game because they had promised a well-developed multiplayer arena battle section and they rushed the game to completion before that could be finished? Did you ever complete the game?
It wasn't a bad game considering they were trying to get it out before the final curtain closed on the Sierra programmers.
I just wish the Coles could re-issue a modernized RPG version adding in all the characters we've grown to love from all the games.
Wiki has a wonderful entry on the game series with lots of info:
http://en.wikipedia.org/wiki/Quest_for_Glory