Programming and designing are two different things. Mario Maker is great not because of its features, but because of its seamless and elegant design.
If you are really making games, you should be able to see that.
Checkpoints : integrating them increases the burden on the uploader (you have to finish the level twice or more instead of once). Plus there are design decisions to make about how much of the world has to be reset to not make levels impossible to finish. And it has to fit their existing palette. At first they didn't think it would be worth the hassle, but reseting the whole world (and the timer) and adding more restrictions to upload (multiple playtroughs) did it.
Info boxes have other potential problems than just (a) being unreadable in different langages.
Imagine a row of boxes above a row of note blocks. This might not seem like a big deal for you, but this is the kind of decisions that makes or break user experiences. Now we know we have to (b) limit their number.
Imagine what innapropriate content they can contain, that (c) can NOT be easily moderated. The current messages and level names are ALL posted trough Miiverse, which has already a moderation system. But the content of the levels themselves are only moderated trough user reports, and currently you can't do much with coin drawings.
There are many more things to consider instead of thinking "can I add that? Then I'll add that" to craft a good experience.
For some, it might look silly to (for example) have exactly three row of items and not one more sticking out (making all new items shakes), or try to fit the mystery mushroom in the slot of the special item and the Yoshi in the slot of the Kuribo shoe. People want to see Yoshi riding Yoshi, but an elegant design wouldn't allow for it. You might be a good programmer, but if you can't see that, Nintendo and you might simply not have the same design philosophies.