Deadly Towers for the NES. I've played through it with the aid of a walkthrough, otherwise there would be absolutely no way I'd have known what I was doing. The game's actually not as horrible as it's made out to be, once you get to a certain point in the game.
They could've made your character more powerful from the beginning, or at least faster. This is a big point of frustration starting out, although you do gain strength and speed through the course of the game. You start out seriously underpowered, and there's really no immediate indication that you power up along the way.
Also, when you take damage, the knockback could've been relative to the direction you're facing. Instead, enemies send you directly downward on the screen each time you're hit, resulting in a lot of frustrating pit deaths. It just seems more like bad design than the trait of a legitimately challenging game. And they could've allowed for a brief window of invincibility after taking damage, like most other games do. Instead, you usually get "pushed" off cliffs due to enemies dealing multiple hits, since you're essentially powerless to defend if you get hit just once. Cheap and frustrating.
They could've benefited from better enemy design, as well. About 90% of the game consists of evil crap blobs and evil spheres. With a castle that looks relatively decent, It's a shame that the enemies are some of the most uninspired you'll ever find in any video game ever.
There's no sense of direction in the game, either. Although I think the cryptic nature can have its charm, I can see how most people would hate it. The game just throws you into the thick of it, with no clue on how to progress. It's never clear what you're actually supposed to be doing...a lot of games from this age had the same problem, but this game takes it to comical proportions. Like the dungeon. God help you if you find yourself stuck in the dungeon. It's the largest and most frustrating area of the game, and you realistically can't make it out without the aid of a walkthrough. It's so huge, but for no good reason. It could've been reduced to a 10th of the size that it is, and it would still be too big.