Most annoying things in stages...

[Captain Phobos]

Venom - The wing at the top of the stage that melee attacks go through, and the flyby shooting ships.
Fountain of Dreams - When the platforms unexpectedly come up from under you.
Kongo Jungle - The music and the shooty barrel that NEVER SAVES ME.
Yoshi's Island - The breakable platform of death in the middle.
Fourside - Good luck recovering from between the buildings.
Green Greens - The blocks with the bombs and the apples.
Great Bay - Good luck recovering under the main platform.
Mute City - That part with the horrible fixed camera angel.
Princess Peach's Castle - Those platform switches.
Icicle Mountain - Too many bloody problems...
Corneria - The ships that do flyby shootings.
Mushroom Kingdom II - The evil pink Yoshi.
Jungle Japes - The killer water.
Big Blue - Ground of death.
Onett - The drunk driver.

 

[KirinKQP]

Considering legal stages, how small Yoshi's Story makes going there for Puff a nightmare, way easier to die. Pokemon Stadium's stage transformations are never good and the ledges for battle field. I usually get shine spiked beneath them and have a terrible time going around them while keeping a good height and my jumps without getting shine spiked again if I mess up. But that's my own problem I can work on.

 

[Dolla Pills]

Brinstar Depths- not tournament legal

 

[Lola Luftnagle]

Well, it would have to be the damaging hazards/constantly scrolling stages, duh. Includes but not limited to:

• Falling tools in Flat Zone that y'all can't watch out for
• The cars in Onett that frequently pass by and do a ****load of damage and knockback. Thankfully, in Brawl and 4, they no longer have the ability to KO.
• Brinstar's sea of acid constantly disrupts proper gameplay
• Klaptraps on Kongo Jungle KO y'all effortlessly if they are are shot out of a barrel and onto the stage; the ones on Jungle Japes can spike y'all in the rushing rapids with no hope for survival
• Random scrolling speed on Icicle Mountain
• Big Blue scrolls extremely fast that if y'all miss a tech on the road, it is guaranteed to KO you.

Now is it any wonder that 80-90% of stages across all four games are never welcome in high-level play?

 

[iAmMatt]

The bombs that fall during sudden death on every stage.

Sudden death would be infinitely more fun if both players could take their time to approach, and every sudden death would be an ass-clencher