Hi I`m trying to think of a vertical scroller type shooter game. But I want to hear other people`s opinion on what makes a shooter game fun? Is it the weapons? the difficulty? the bosses? Uniques features?(ie. weapon switching or enemy capturing ) Comments about shooter games are very welcome.

The animations, the sounds, the explosions, the projectiles that you are firing. For the animations, it'd be cool if enemies could fracture into chunks. The enemies - if they could respond to your movements that could improve replayability.

Fast action - thats what I look for when I decide to play a scrolling shooting game.

There are many different styles of shoot'em up and tastes of shoot'em up players, and they meet in very strong and narrow preferences: whatever you do, many won't like your game.
Apart from novel and experimental features and combinations of features that should be enough to consider your game interesting, to make your game disliked as "well made but I prefer something else" rather than disliked as "some incompetent's waste of my time" you can concentrate on subgenre-neutral aspects like production values and general game balance concerns.

Minimum requirements for production values include decent graphics, music and sound effects, an adequate game length, reasonable playtesting of levels, no serious bugs, etc.
Game design errors that you are supposed to avoid or correct include:
- requiring too many fingers and hands (or too many simultaneously pressed keys on a PC keyboard)
- rewarding easy or boring strategies (for example overwhelming firepower might reduce enemy shots to the point of making dodging unnecessary)
- relying on impractically complex or tiresome gestures (from mashing the main button to shoot faster to drawing shapes with the mouse cursor)
- positive feedback that ruins game difficulty (for example, if firepower powerups can be collected and improve survivability most of the game is likely to take place at full power without dying)
- unfair and inconsistent challenges (for example randomly distributed ramming attempts from behind, that cannot be memorized, or confusingly similar appearances for different enemies)

It needs intuitive controls that people won't have trouble remembering, using only two hand positions, for example, using arrows to move and Spacebar to shoot with maybe only one or two other keys, like A to drop bombs or whatever.

The graphics should probably be smooth, I've found that game get a higher score if they use smooth, simple sprites (or whatever your using) instead of pixelated, complex ones.

Also remember to make enemy's that are very different to each other look very different to each other, so players will instantly tell them apart and know what to expect, unless you want it to be confusing of course.

Finally I would also try and make the effect's such as lasers, bullets, shields, power ups, etc easy to tell apart from one another, as most players find it extremely annoying to be killed by something they thought would only wound them, or to risk their life's for a health boost only to discover it was extra ammo for your weakest gun.

This idea is fail.

personally, i would play a scrolling shooter, of like a tree side scrolling shooter, between a jungle gorrilla (WW2 not the animal) facing off against ground forces, its original, i came up with it in 30 secs and i bet it would be easy to do :P id love that

Scrolling shoot-em-ups (AKA: SHMUPS) is a nitche genere with a hardcore following. I'd suggest you google "SHMUPS" to find out what the potential audiance likes as there are a number of great websites out there dealing with the genre.