I think it's a good idea, but some hazards should stay, or you should be able to specify which are on and which are off. Consider this:
Imagine Mute City, without the cars, and without the main platform moving.....hmm......that's the dumbest stage I've ever thought of, which makes me think they should keep something like the platform moving, but no cars, if they did that. But making each hazard an option would be best.
Also, while I agree that if all hazards were removed from the game, a huge chuck of the stages would be exactly like Battlefield, they DO have their own mood. Try playing on Battlefield, then Yoshi's Story with a character of your choice, hopefully the cloud and Shy Guys wont distract you too much, lol. You might notice however, that you play differently...I know I do, I notice that in Battlefield, the mood is totally serious, while in Yoshi's Story, is it more cartoonish, the music also effects the way I play a little bit, but that's just me. Sometimes music gets boring after playing a level too long as well, so that would be another reason...
Another thing I think should be optional is Pokemon that come out of Poke balls, and Characters that come out of Assist Trophies. (Considering that some, like Celebi, are really important, and are supposed to be rare, and hard to get the first time, this should only be available once you've seen all of the Pokemon fairly, and if only Celebi was selected for example, the points given would be removed....then again, this could still run into problems...we'll see....I'm sure Nintendo will make the right decision anyway) But then again, I'm not sure how these Assist Trophies will work yet at all, because I was thinking, and maybe there's a possibility that maybe you can only get a certain Assist Trophy once you unlock a trophy of it. I don't know though...