The tower bosses are quite basically like the bosses as they were in SMB3. The world end bosses are the Koopa Kids again, but the structure of the boss battle is changed to make things more interesting. They're pretty easy for the most part, but still fun to play. And yeah, they're really not a big part of the game.I'm also glad they brought back the Koopa Kids, I just hope they don't overuse them. They're kind of samey, is the problem. They've got different attacks, but they all fall under the same basic pattern, and they all have the same kind of basic form. Meh, who plays Mario games for the boss fights anyway? I'll live.
The game itself actually is pretty challenging, but in single player there's so many opportunities to stack up lives that you really don't have to worry about reaching the game over screen as long as you have a good measure of skill.
Speaking of which, who else has reached 99 lives? ;)