The secret sauce of game replayability

I was reading an interesting article on destructoid the other day about the endgame syndrome, or why we abandon games (either half played, or not) and I disagree slightly with the author on the main issue. The article points to two reasons why: that either games aren't as compelling as they used to be, or gamer attention spans are decreasing due to a variety of factors (different age group? many more games than 10 years ago to pick up and choose from...). While these are true, it still doesn't explain why some games still have the "magic sauce" that make me want to complete them and keep me coming back to that game over and over again.
I'm more of a one game person, I rarely cheat on the game I'm currently dating. I may drop that game to check out another, and play that completely before coming back to my favorite one but playing two games at the same time feels like I don't do justice to either of them. I don't spend the workday thinking up strategies to try out when I get back to my games after work, or read up on the gaming forums on that game since I'm not fully absorbed in one.
But what is this magic sauce? Some people say that games are either too easy or too hard, but I found Ninja Gaiden (which as classified as too hard) enthralling and I played it all the way through and then repeated half again to get stuff I had missed the first time around (including the hidden original Ninja Gaiden games that were in an arcade machine in the game - nice touch :) ), and I found Obi Wan, which was an easy game, very (re)playable, I finished it and then went back and ran through again a couple of times. And then there are games like Halo2 and WoW which I played endlessly, they became my core games that I occasionally left to go check out new games, but always came back to (the multiplayer, not the Halo2 single player which I did finish, but only once).
So what is the secret sauce that made these games so playable? I have some theories about what makes these games good for me, but I'd love to hear what you think and what keeps you hooked on a game:

• Character progression: By this I mean the timeline that allows you to build up your character to become more and more powerful, by discovering new talents, new spells, new weapons and in some cases new personas. The game never gets old as I start using the new abilities of my character as I level up, and gives a feeling of achievement as my character swats down mobs with ease that were at one time a threat. Warcraft is a great example of this, but so are so many other games like Fable where not only does your character get more power the more you play, but even the reactions of the townspeople change as you gain reputation.
• Multiplayer: This is usually online, but whether co-op or adversarial, this is a big part of game re-playability for me. I played Rainbow 6 to death, playing the co-op missions online with both my friends as well as PUGs online. The fun of coordinating a room entrance with 3 other people and finishing a mission despite 3/4 of the team dead was an achievement for the whole team that didn't pale for a long time. The Halo series was so popular due to it's fantastic online multiplayer support and the countless hours online with friends was well spent, despite the hordes of 12 year olds who had just discovered profanity and tried their best to grief you out of enjoying the game. The saving factor here was that as long as you didn't get into a PUG and played with like minded players on your team, the online experience was fun.
• Good strategy requirements: This refers to both classical strategy games such as Age of Empires or Command and Conquer, as well as strategy in games like Halo, WoW or Rainbow 6. Having to coordinate with other players, or even just laying out a strategy for yourself against the computer is a lot of fun and then trying to either tweak the strategy or find new ones, as in the case of AOE, or just getting your core strategy down pat, as in the case of Warcraft, was always a lot of fun.

I'd love to hear other thoughts on this, please feel free to use the comments section below.