By hard, I'm thinking the stages themselves. I don't think I've ever had so much difficulty in any of the first six games. And I've never, *never* needed two continues to even reach a boss before. But the missing abilities like sliding and charge shot do contribute to the increased difficulty. For example, the falling boulders at the mini-boss in Jewel Man would be a lot easier to avoid by sliding.
But honestly, I think it's mostly because they *wanted* to make more difficult levels, which is nice. There are a lot more spikes, more bottomless pits and more tricky obstacles than before. And you can't keep moving forward without thinking like in the old games, because here it gets you killed fast.
Frankly, there are only two stages of old I found harder than those in this game: Heat Man for the disappearing bricks (even worse than Plug Man's) and Ice Man for those buggy platforms that would hurt you (and then let you fall to your doom) if you happened to land on them while they were firing. Of course I'm not including fortress stages here, because some of those have managed to be really tough, but I expect MM9 to kick my ass in that department too.
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Current projects: Metal Storm (NES), Clock Tower (SNES), Ristar-The Shooting Star (Gen), Sonic The Hedgehog (Gen), Sonic CD (Sega CD), Mega Man Zero (GBA), Battletoads (NES), Bucky O'Hare (NES)