Something many people seem to forget when they're trying to recapture the vibe of the "Good ol' days" of platforming is that even those games had some sort of story(albeit often laughable) to give you some motivation to aid the protagonist. Given that Castelvania and Ghosts and Goblins were specifically mentioned as inspiration, perhaps inspiration should be taken from those games in this regard as well.
Overall thoughts for potential improvement:
- Story: Doesn't have to be much but much like journalism, some thoughts into Who, What, When, Where, and Why can generate interest. All five questions don't need to be answered but they can help. Castlevania introduced the Belmont clan and the quest to kill Dracula, G&G had Arthur and saving the Princess.
- BG music: Although I'll admit, I'd rather a game have no BG music over crap music.
- Crouching attack should exist, especially since there are enemies at that height.
- Upgrades: Again looking at the supposed inspiration for the game.
- Graphics: Blocks are kinda jarring against how the character/enemy designs look. The player, and enemies look much more detailed. Traps are another thing, throw some variety into them. Instead of say just using the arrow trap for all of your horizontal projectiles, add a fire ball that perhaps doesn't shoot as often but can't be ducked. Spinning blades could also be done with saws, rising spikes, etc. Variation keeps people from getting bored and by merely changing the appearance of the trap while keeping the effects the same, many won't notice it immediately.
- Hook: Why should people play this game? Especially in this day and age where it's rather simplistic to find a way to play the games(Emulators, Wii-Ware, etc.) you're looking to emulate, you need to not only try to recapture the feeling of those games but again give someone a reason to play YOURS.
- Level Design: Pathways should not be designed in such a way that it visually blocks part of the character. Any of the low ceiling tunnels(for lack of a better word) clip the player's head. This should be rectified.
Bugs:
- Player is able to throw through single brick walls if pressed directly against them. Does not work while jumping.
Final thought: While my opinion, the, "One, two, fuck you" design philosophy was made primarily for arcade games(which G&G was originally) in order to get players to keep feeding coins into the machine. But even then, the games were enjoyable and didn't bore players. I'd recommend keeping this in mind due to the fact that your medium(flash game) has plenty of competition floating around that cost a player little more than a few clicks to switch from your game to another.