Crystalis/YMMV

Anticlimax Boss: You'd think the actual endboss of an SNK game would be an SNK Boss, but compared to Those Three Or Four Bosses and the annoyingly hard stage you go through before the final battle, DYNA is almost insultingly easy.
- Averted in one of the few good gameplay changes in the Game Boy Color remake - Draygon is fought after DYNA.
Crowning Music of Awesome: The whole game (at least the NES version) is made of this, but especially the music for Fort Shyron. You definitely get a feel for the residents' desperate struggle against Draygon and his army.
- Wild Fields
Demonic Spiders: Dear Lord, the wizards in the Pyramid Basement right before The Very Definitely Final Dungeon. Not only can they hit you with a spell that turns you into a helpless slime, but their elemental resistances are set up so that you have to take them out With This ~~Herring~~ ~~Butter Knife~~ Sword of Wind. Oh, and did I mention that they have craploads of Hit Points? Good luck getting to the rematch with Draygon without burning through your supply of Fruits of Repun and using all your magic points flying in a (sometimes vain) effort to avoid them.
Game Breaker: The Warrior Ring, which enables you to fire off the first level of your charged attack with just a button tap. You don't get it until the endgame, but it allows you to rip through anything vulnerable to a level one blast from the weapon in question. Note that this does work with the Crystalis sword, and the final boss is vulnerable to this, which is a big part of why it's an Anticlimax Boss. Notable that all of the bosses immune to this all fall under That One Boss. In the final battle with DYNA, Crystalis can do that naturally, making even the one level of charge the sword has irrelevant.
Genius Programming: For its time, this game was a surprisingly advanced NES game, such as its art, gameplay, or animations. It can easily put quite a few SNES games to shame in that regard.
Goddamned Bats: Bats, birds, harpy-looking things... really, any enemy in the game that can fly. Near the end of the game, you get Toxic butterflies that, upon being killed, release clouds that poison you if you touch them. Once you get the Warrior Ring and armor that has immunity to poison then you're mostly safe, but then the helicopter droids in The Very Definitely Final Dungeon can get you just from the sheer amount of damage you take from them and the fact that they're everywhere.
- The wraiths that appear in Sabera's Castle and onwards, which are invisible and invincible unless you use a Tornado Bracelet attack on them. Trying to stop and kill every single one means you're going to run out of MP really fast, but running past them leaves you wide open to the paralysis beams they fire. They also have an annoying tendency to completely obscure their shadow behind the bottom wall, meaning it's possible to accidentally slam into one without even knowing it's there.
Porting Disaster: The Game Boy Color port of the game, released a decade later, makes alterations to the game mechanics that make things harder than the original, among other things. To some, the complete sundering of the plot to something much more basic and boring is even worse.
That One Boss: Pick a member of Draygonian Empire (Well, Kelbesque and Sabera aren't too bad.) The rematch with Mado in Goa Fortress is particularly noteworthy for the same reason Quick Man is noteworthy: he bounces around the room much too quickly to be hit reliably while happily plowing into you in return.