Seeing as I had no real options to play Mega Man 9 (or 10 for that matter) I took the lazy man's way out and watched some longplay videos.
Even then I could draw some conclusions.
MM 9 is doing its best to try to look and feel like a NES-title, but without any of the hardware limitations. Any limitations imposed are artificial, solely towards the goal of catching the look and feel of the NES-titles. No slowdowns ever happen, no flickering is ever visible. I don't mind that one bit because then MM 9 'cheats' a bit by doing things within those limitations that go unnoticed to the untrained eye.
Platforms that only move based on MMan's momentum, platforms that spin MMan around, enemies moving at super-speeds, lots of things going on at once without any slowdowns... and some really cool bosses. Thematically the Robot Masters are sillier than ever, but how they act and fight in-game is far from that. More importantly their weapons are really awesome, despite their silliness. There's a Robot Master who fires bees as a weapon. The only thing funnier about that is that the first ever female Robot Master in the series happens to be vulnerable to the bees! Tornado Man fires a tornado which sweeps all non-boss enemies off the screen, making it one of the most powerful weapons in the entire series.
Then there's Galaxy Man, the Robot Master who fires black holes.
When you consider the story for a second, it's about Robot Masters rising up because they're all gonna end up on the scrapheap. I honestly don't know which thought is worse - that there's a Robot Master out there with the capability to create black holes at a whim, or that such a robot will ever become obsolete and needs to be decommissioned.
It's at this point I realized that the devs are gunning for comic absurdity, so I just roll with it and enjoy some laughs.
But seeing all these cool gimmicks and tricks made me realize just how restrictive the NES-hardware is. It's not because of lack of creativity that we have 48 Robot Masters, of which three use a time-stopping weapon, four use an orbiting shield, three use a returning projectile, and almost half of them jump around the screen like acrobats - it's because the hardware didn't allow for much more than that. When one studies the more abstract Robot Masters (just to name half of the boss encounters in the games) one begins to see the creativity at work behind them. The best contrast between the old and the new here is the orbiting shield. The shield functions almost identical in all four instances it appears in the NES-games, but then check out the Water Shield in MM 10 and how it's vastly more versatile.
One major addition to MM 9 is an item shop. Screws are now a droppable item which counts as currency. Take them back to Dr. Light's lab, and you can buy useful consumables or a handful of upgrades - and even a cosmetic item to boot! The Energy Balancer makes a return to the games via the item shop, but again I must ask why it wasn't just included from the start.
But at the end of the day, MM 9 is trying to be just like the NES-titles of old - and succeeds quite well.
MM 10, on the other hand, uses MM 9 as a stepping stone and tries new things. In its full mode MM 10 now offers a choice of three different characters to play, with three different weapon sets and playstyles. Lots of replay value there. But what I really took away from MM 10 was how the humor is cranked up a notch. I laughed out loud at Sheep Man just by seeing the name, and again at his weapon name. Strike Man deserves a special mention for his 'theme', and I almost fell out of the chair laughing when the map showed the path to the final Wily-stage. But at the same time MM 10 is filled with humor, it's also quite snazzy at times. Nitro Man is easily one of the cooler Robot Masters so far, and Commando Man's 'well-'ard' appearance is only one-upped by his weapon, one of the most versatile bomb weapons the series has seen yet. Then for one of the castle-stages they revisit the element of reusing Robot Masters from older games, but this time they get it right.
At the end of the day MM 10 is trying to 'evolve' into a more modern game, while still trying to appeal to nostalgia fans of the series. It's a direction I don't exactly approve of, but I can't deny that the game has plenty of content and feels fun to play.
Then I made the mistake to checking out MM 11 - Nope, not my cup of tea.