It's easy to see why people are so motivated to play these games...they have complex and highly responsive reward systems as you describe.  Although my extensive work with computers has probably reduced both the motivation and time I have for games, there have been a few that have captured my attention and become favorite pastimes...chronologically they would be Zork, Cruisin' World, Shadows of the Empire, and Goldeneye...guess that dates my prime 'video game period' pretty accurately LOL. More recently I have spent considerable time in Second Life, originally because I was researching it for inclusion in COMP 214, but also becasue I have enjoyed meeting people from other parts of the world, learning to build, performing live music shows and developing an SL clothing business (curling sweaters, beaded rawhide and burlap sack dresses...a niche market if there ever was one).

Of course, SL is not really a 'game' in the strict sense of the word, although in business and entertainment pursuits there is an element of competition I suppose.

I have also played Zork, Cruisin' World, Shadows of the Empire, and Goldeneye. Great games. This last 3 games are almost a who's who of games on N64. 

Goldeneye is an another excellent example of a game that fosters social interaction between the players with a very competitive edge. This game brought the concept of multiplier to its heyday for both hardcore and casual gamers.

Today we don't see this much interaction in games as we are separated by miles of Ethernet cables. I recall a day when a case of beer, an N64 with Goldeneye and a box of wings was a catalyst for an amazing leisurely experience.