I downloaded Megaman 9 on virtual console yesterday, and I have been playing it quite a bit since. I have already managed to defeat all eight Robot Masters, and have made a couple of attempts on Wily's Fortress. There is something very nostalgic about playing a game deliberately made in the image of games from twenty years ago. The similarities go well beyond graphics: Megaman 9's gameplay is firmly based on the early Megaman games. However, there are also clear signs in the gameplay that Megaman 9 is not an actual NES game, and draws upon innovations and developments made in later entries in the franchise.
One way that Megaman 9 does strongly resemble the games it was based on is in its stage layout. All eight main stages in Megaman 9 generally scroll from left to right as the player advances, with occasional stretches of having to climb or descend vertically. There are no large maze-like structures, alternate rooms, or separate paths branching off from a central hub, which are all somewhat common level design features in later entries of the franchise. There are also no stages built around an unusual form of transportation (first appeared in Megaman 6, but popularized by X4). So the overall flow of the stages feels just like it did in the original games. One design element in particular that gave me a strong sense of nostalgia was having to jump down into pits in order to advance onward, going through a single intermediary room before reaching a new horizontally-scrolling area. I haven't seen something like that in any of the recent games.
Perhaps the biggest way that Megaman 9 emulates the original series is in its enemy AI, particularly in the way bosses fight. As the Megaman series advanced, bosses tended to have more attacks, more distinct weaknesses, and much more complicated AI routines. Megaman 9 returns to the era when many bosses just ran back and forth in their room, jumping and spamming their one weapon (ironically, these kinds of bosses are much harder to fight than smart ones). Unlike in later games, none of these bosses change their attack patterns and capabilities based on their remaining health or if they are hit by certain weapons. Finally, the only sign that a weapon is effective is that it does more damage than other choices, unlike in later games where a boss would often have a special damage animation.
However, there are a couple of ways in which Megaman 9 is clearly a later installment of the Megaman series; most notably, the addition of the shop and special challenges. The shop is clearly at odds with the 8-bit design of Megaman 9, since only Megaman 7 (SNES) and Megaman 8 (PSX, Saturn) have previously had shops. However, the addition of the shop itself is pretty, minor; it is the presence of bolts (currency) in the stages that really stands out. In particular, the sound made by game when Megaman picks up a bolt is not one of the sounds found in the original 8-bit games, which actually threw my brother off a couple of times (once, he thought I had picked up an extra-life instead). Of course, the challenges also stand-out as being new, since they are a completely new addition to the series.
A less obvious, but much more significant, difference can be found in the presentation of the game's story. The opening cinematic of the game is itself much longer and more involved than is typical for an old-school Megaman game. What really stands out though is the presence of several cut-scenes that occur as Megaman vanquishes the Robot Masters, in which Megaman recovers pieces of the defeated Robot Masters to be analyzed. This kind of sequence first began to appear prominently in the series in Playstation era. Furthermore, the game's story also involves the morally ambiguous fact that robots get shut down and dismantled after a certain term of service, a plot element that is much more similar to entries in the X and Zero series than the normally happy and carefree original series.
Overall, while Megaman 9 embraces the design of the earliest Megaman games, there are still tell-tale signs that it is a later product of the franchise. So Megaman 9 really represents a really interesting hybrid of the new and the old. With the apparent success of Megaman 9, I am curious if other franchises will be given this kind of retro treatment.