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1 Super Mario World and Super Metroid are both flawlessly designed from the ground up for ROM hacking in basically every possible way, "almost" as if it was actually intentional on Nintendo's part
Seriously, who needs Super Mario Maker when you have this
2 Mike Tyson's Punch-Out on NES has a completely random dude in the audience that indicates exactly when to punch certain enemy boxers for an instant KO
He doesn't look a thing like Jesus, but he may or may not help considerably in the final boss fight against Tyson/Dream as well, so pay attention
3 Undertale has its save system tied extremely tightly into its storyline and even remembers every single thing you do in it between runs; also, the character Flowey himself is a giant meta-commentary on this
Even though it's been talked about to death and beyond,
I'm definitely not the first to admit that not a lot of people seem to really appreciate how absolutely massive of an impact this really does end up having on the overall game as a whole
4 The infamous "NO, NO, NO" boss from Radiation's Halloween Hack hints at the player being able to run away from it by having the battle-introduction screen transition and music for standard enemies
Also, attacking its crotch (the source of its manhood) deals an extreme increase of damage to it when compared to its other parts, assuming that the player actually IS stupid enough to try and fight it normally
5 Metroid Prime tricks uninformed first-time players into thinking that the Great Poison which the Chozo Lore entries in its Chozo Ruins refer to is the nasty green toxin that the area's boss distributes into its water, rather than Phazon
Still a pretty great AMOUNT of poison for one boss to be distributing through its own body if you ask me
6 In Sonic CD, teleporting into the Bad Futures not only makes their corresponding areas broken-down and pathetic but also the enemies themselves, to reflect how carelessly Eggman has treated them
Projectile shooters' firing devices no longer work, the spikes on formerly spiked enemies become nonexistent, the jumping spiders in Wacky Workbench no longer deploy webs, nearly every single one of the Bad Future enemies looks like it is just about to completely fall apart altogether, et cetera
7 In Super Metroid, water from Red Brinstar and Maridia leaks its way down through Planet Zebes' inner crust into one of Norfair's Grapple Beam tutorial rooms
Mostly just another excuse for Nintendo to remove almost any possible semblance of actual challenge from the vanilla game, but still deserves mentioning just for how incredibly meticulous of attention to environmental design detail it really, REALLY is
8 Playing on the Game & Watch stage in Super Smash Bros. Melee converts your characters into 3D imitations of 2D side-scrolling sprites
Also, Mr. Game & Watch himself is in fact one of these
9 When you defeat Nick Bruiser, the terrifyingly muscular and brutish final boss of Super Punch-Out, one of the names listed on the leaderboard for best times on his defeat is Glass Joe, possibly explaining why the character is so mute and depressed
Also, Joe then carries that win over into Punch-Out Wii, possibly contributing to his sudden MASSIVE confidence upgrade upon receiving his new protective headgear in Title Defense
10 In Wario Land 4, most of the background music is programmed to dynamically alter itself in basically every way possible for the Game Boy Advance hardware
Crouching: Slows music down
Rolling: Speeds music up
Status Effects: Alter the pitch AND speed of music in hilariously bizarre ways
Much of the music itself: Is amazingly offbeat and bizarre