List of easily KO-ing items:
Bob-Omb - Shieldable, Dodgable, Catchable
Final Smash - Situational, stage and character dependent.
Fan - Predictable, Can be positioned to make it less effective if it does hit, can be DIed out of, situational based on stage and character matchup.
Curry - Situational, stage depended, reflectable, makes every container and bomb spawn a personal danger while active.
Smart Bomb - Can easily be DIed out of, leaving you both on safe ground and having taken less than 15%, rarely if ever KOs
Starman - Run away for 10 seconds, possible on all stages, downright easy on most.
Hammer - This item is such a joke to competitive players that they rarely risk picking it up. The person holding the hammer is at great risk of items being thrown at them while they can do nothing, being projectile locked while they can do nothing, clashing and losing the hammer head while they can do nothing, being perfect shielded and thrown, at which point they can do nothing, otherwise its absurdly easy to just ledge stall.
Golden Hammer - A little harder, but many of the techniques that make the black hammer such a joke apply here too, and it is one of the few items with a long spawn animation, so it never really 'appears in someones lap' without warning.
Bumper - Blockable. Travels only a set distance.
List of Items that drastically change the way a character or all characters are played:
All of the above
Screw Attack
Franklin Badge
Giant/Poison Mushroom
Metal Box
Sticky Bomb
Motion-Sensor Bomb
Party Balls
Blast Crates
By introducing items, it turns Brawl into a different metagame. That's not necessarily bad, but it would not replace the current metagame. It would exist alongside it, if it becomes possible.
That said, the luck issue still needs to be addressed.