Tobold talked yesterday a bit about the fun involved in learning and mastering new skills. His basic thesis is that people have a lot of fun learning skills but they get bored and stop having fun once the learning stops. One of the things he calls for is people making new and interesting games that are extremely different from what has come before to avoid the problem of early mastery. I certainly think that there is a tremendous amount of transferable skill from some games to others. Many games of virtually any genre have substantial skill crossover from MMOs to shooters to turret defence. I can say for sure that the reason I was able to start playing Mass Effect on Veteran difficulty is because I spent so much time playing WOW and Doom 2. Strafing in and out to snipe enemies and hitting lots of skill buttons without looking away from the screen are skills that easily go across games and reduce the learning curve a lot.
I am not so easily convinced that games need to have truly revolutionary mechanics to be tremendous fun though. I finished Portal 1 and then played Portal 2 and I had a hell of a good time in the second game. There were a few new mechanics but I didn't feel that they really added a huge amount to the game - the structure and ambience was noticeably different and I really enjoyed the newer, bigger areas to play around in. Mostly though I really just wanted more of the same and exploring through the game was awesome. I think entirely new mechanics can add a little bit more to a game's longevity but eventually the player maxes out the skill needed to beat the game's challenges and then exploring is the only thing keeping them going.
Ratcheting up the difficulty of the game can actually be a really good way to change the mechanics. For example, if you are just playing through the levelling game of WOW you can easily get to the level cap with only 3 abilities on your bar. This isn't optimal in any sense but it is easily doable. When you get to raiding you will suddenly realize that you simply have to use more abilities and learn new techniques. I was good for a long time at jump spinning to get extra attacks in on mobs while running but I really mastered it while fighting Sindragosa hardmode. That fight was absolutely full of running away from the boss and learning to use all of my abilities in the correct order while running was a challenge - they all had different GCDs and ranges so a lot of expertise was needed to maximize my damage. (Whether or not this maximization really led to significantly improved kill rates is questionable!) Despite having played WOW for 4 years at that point I was still learning because the fight was hard enough to force me to learn.
I feel the same way about Mass Effect. While playing on Veteran I pretty much ignored all of my abilities because I was able to get by with just strafing, ducking behind cover and shooting. When I ticked the difficulty up to Hardcore I suddenly could not get by; I had to precast shields, use my crowd control, manage cooldowns and retreat more often. I also had to read all of my abilities and make more careful choices when levelling up. I learned more and had more fun because the game was hard enough that I had to. Just like doing WOW hardmodes required me to learn entirely new things like doing 95% of optimal DPS while still watching boss mods, cast bars, fire on the ground and other environmental signals there is an entirely new set of skills just waiting to be mastered when the challenge level goes up.
I definitely think that revolutionary games like Portal have a much easier time of creating fun because they don't have to ride that edge of hard but doable so much; the novelty creates the fun. When a game is created that uses established mechanics and which people can easily apply old skills to I think it is much harder to be fun because you need more content or more tightly tuned challenges. Bringing innovation to the table can really lower the bar for the rest of a game but it isn't required; give people tough but fair challenges and a world to discover and it can be fun just the same.