EDIT: I will also confirm that the Wily 3 boss is a humongous asshole.
I hope you're still talking about a Buster Duel situation, because he's stupidly easy.
Already beaten this game, of course; took me an hour and a half my first time through. I could probably do far better now, too, as I wouldn't be wasting time.
Thoughts...
Music: It's different from MM9's. I need to really sit down and pay attention to it. While MM9 tried to be upbeat and catchy, MM10's seems to be more dreary and moody, and I actually do rather like it.
Weapons: Okay, yeah, a good chunk of the weapon selection sort of sucks for stages... but the Triple Blade and Water Shield are stupidly hax-tastic. And the Commando Bomb is fun, if sort of limited for speedrunning use.
I feel like noting that one thing I really enjoy about the weapons in this one is that using them against the bosses isn't generally as simple as 'run up and shoot them with it' - in fact in many cases this makes them work WORSE (
).
Bosses: I found these guys to be a lot more fun that MM9's because none of them are pattern-based; you really do need a lot more reaction and skill to do it, it seems. Weapon Archive was nostalgically awesome, etc.
Little things: I like the change in Challenge system, or would, if it had a bit more variety sadly (See: Mega Man Powered Up). Ah well
Bragging Rights Moment:

I'm the only person to clear the Boss Rush stage in Time Trial so far >8D ...It's still hellishly irritating though.