I wouldn't really put super mario galaxy into any kind of a hard category. I had a few friends trading off with me through that, but we still managed to get every star in under a week. Deaths were more from just screwing up and were few and far between.
In response to both you and the GP, there is also a difference, at least in my mind, between the difficulty to beat a game and the difficulty to to achieve some kind of 100% completion. I've felt quite a few games have been easy to complete, but spending hours hunting down some hidden object gets to be both annoying, and frustrating.
Mario Kart took a few hours to get used to using a wiimote, but after that the mirror cups were not that hard to beat. With a gamecube controller the mirror cups were easy, well, unless you count random blue shells 10ft from the finish line as a difficulty. I don't really consider the staff ghosts part of the games difficulty, though they are incredibly hard to beat (and yes I did manage to beat the expert staff ghost on rainbow road).
The New Super Mario Bros is definitely a challenge though. I haven't had the time to make it all the way through that yet though. But that game is enough of a challenge even without needing to collect all of the 3 large coins in every level.
Even games that are usually considered to be hard games have a difference between being hard and having 100% completion. I've managed to beat both Halo and COD4 on their hardest difficulties and enjoyed the challenge. However, spending hours hunting down skulls and intelligence pieces and completing every achievement is not something I would enjoy spending hours doing. Beating a game is one thing. Spending hours hunting down some arbitrary challenge just so your screen says 100% instead of 98% is another entirely. Even though I have spent the time to get that 100% completion on some games, I usually don't factor extra challenges in as the majority of gamers, even hardcore gamers, are often ok without extra random challenges.