Oi, what did I do? Oh well, I might as well continue on.
I think New Super Mario Bros. failure to do new and cool things with powerups is because the game was intended to be like the original - the new powerups including the shell are all basically extensions of original Super Mario Bros. concepts rather than truly new ideas like flying, riding Yoshi, etc. This makes it inferior in some people's eyes, and I understand that but I think it made the game easier for anyone to pick up and play, not just old-school hardcore Mario fans, and the level designs made up for it in my eyes.
Overall I liked the game better than Super Princess Peach because Super Princess Peach's vast array of gigantor levels got on my nerves. In 2D I prefer quick, linear levels with lots of action whereas Peach was making me explore. I also felt Peach's levels all started to feel the same after about 2/3 of the game was up, while New Super Mario Bros. kept delivering new kinds of levels right to the end. Peach's level structure quickly started to feel static: explore, collect, explore, auto-scroll, explore, collect, explore, auto-scroll, explore, collect, explore, etc. Mario was free to be something different in each level.
Super Princess Peach was a deeper game, for sure. I just didn't think it was as much fun.