Well, I have quite a few I think.
1: Compare Kingdom Hearts 1 to Kingdom Hearts 2. I know people that think the first one is actually very easy. The first one does offer you very few options really to kill your enemies, compared to Kingdom Hearts 2. However, I find the first game rather frustrating as a result. I don't quite understand why, but I just die a whole lot more often and I don't really know what to do about it. That might be because I chose to sacrifice defense. Yet, in KH2 you have things like drive abilities, limit breaks, and reaction commands to tap into. These do add a lot more options for dealing with any situation, so there's a lot more I can actually think about if I happen to be underleveled or very low on health or defense or something. Not as if either game's really that hard.
2: Borderlands, kind of? I find the hardest part of the game potentially is before you actually have a shield and your action skill. The only real option you have is shooting stuff, and if you don't do that really well, you're just doomed. The more skills you get, the more options you have. Shame that the entire game's really simple though.
3: Warriors Orochi vs. Dynasty Warriors. Sometimes DW is legitimately a bit tough because you have to place a lot more consideration into simpler things, like positioning, rather than doing cool combos. In Warriors Orochi, the game throws a loooooot more at you, but you gain a very large capacity for actually setting up combos to deal with it.
and I guess in terms of more substantial games... I do really prefer AI War to most other RTSs. Again, it's really just because you have so many options. You think, right, I can't send my fleet in because of the Eye. I'll use my starships. Oh wait, there's an orbital mass driver. I can mass produce fighters and send them in one at a time to lure all the ships to the wormhole, and set off an electric warhead. Then you, um... well, I don't know what you'd do. But, well, I could certainly spend an awful lot of time coming up with a great plan with the large amount of options available, even though the AI cheats. When a game is much simpler, I guess I can kind of cite Starcraft 2, there's a whole lot of focus on positioning, on manipulating your forces and using your abilities to the exact pinpoint fullest potential. You have to actually focus on nailing all of the tiny details in a simpler game, rather than coming up with a clever solution given a large set of tools.
Another good example, I feel, is Mark of the Ninja. If you just focused on your sword and running around stealthily, you'd have a tough time. The game would probably be impossible by a certain point, unless you at least use your darts... but the more tools you get, the more clever you can be, and the game just becomes easier. In fact, the game becomes much easier when you realize you can use running around to make noise as a potent distraction tool. I like having all of these options, because I just seem to utilize them better. The fewer options there are, the more precisely you must use them.
Edit: For the record I feel a tiny bit self-conscious actually for listing off all of these action-y games after everyone else lists off complex 4x strategy and stuff... although not self-conscious enough to really be too concerned about it.
Knights of Pen and Paper, I played a bit, and I actually don't like it at all. It's too simple. It's so simple and it's really easy as well, at least as far as I've experienced. Take quests, fight stuff, take quests, fight stuff, remember to use leader strike and stack all the healing on the paladin, lah lah lah... The fighting stuff wasn't particularly fun either. Around the 500th time I meteored an entire enemy party to death, I just shut off the game and it didn't get a second session. I was often fighting 3+ enemies at a time as well.
A lot of turn based games that are similar are actually already fairly simple. Mash attack during random encounters, use the boss's weakness, and heal when the boss uses a largely random attack from its pool. There are few I can think of that make you actually use the many options you have, the two I can think of being Final Fantasy 9 and 10.