This post is relevant to my interests.
These are some vague questions, so the best I can offer is vague answers. Not to mention they're going to have some of my own bias included. 
I'll just take this nice list
-What would add replay value? Megaman Zero saga: new game+ bonuses
Super Metroid: sheer atmosphere and exploration breadth
where the fuck is the jump ball? I swear I never found it!Metal Slug: I want to free ALL the POWs so if I lose a life I reset.
Demon Crest: Variety of endings. Metamorphoses of the player character.
Different characters to play the game with. That's awesome.
-How many levels should there be? Huh. Hard question. I don't know, but please, no "whatever previous level revisited" or at least try to avoid it.
-How difficult should the game be? Either hard to survive or give the player incentives to try and go for perfect runs.
-What are the good things you've seen in shooting platformers?
Eye candy.
Super Metroid: Again, sheer freedom of exploration. Arcane powerups.
Metroid Fusion: the SA-X
Megaman Zero: Charismatic enemies. Interesting poweups that change the way you play.
Demon Crest: A lot of bosses and phases seem outright impossible until you manage to figure them out. Arcane powerups. Really manages to keep you "in the dark" about what's next or how do you beat the game, in a good way. Playing a bad guy.
Cave Story: Heart-wrenching drama coated with cute sprites.
-What are the bad things you've seen in shooting platformers? Super Metroid: The backtracking. Oh Glob the backtracking and checking every corner in search of that itty bitty breakable block that'll take you somewhere
essential.Negative counterpart of exploration.
Megaman Zero: Saves. Lifes. Continues. Scores. Score ratings. GGGAHHHHHHHHHH

And grinding for energy cristals / XP / health. What the fuck is GRINDING doing in a game like this? at least grinding in some castlevania games is moar liek gambling since you're going for a rare drop.
Demon Crest: Arcane game sequences to see all the endings. The negative of being kept in the dark in a good way is, well, just being kept in the dark.
-What would you like to see in the future from this genre?More compelling stories (cave story) and drama, character development. More and better narrative overall. Meaningful choices in powerups, not just accumulating them (Cave Story has this, so does Gunstar Heroes, but in different ways). Game-long choice of player character among several. Optional / mutually exclusive levels.