Heavy Metal doubles the weight of the Pokemon that has the ability. This is mainly a negative ability as you would take twice as much damage from Grass Knot and other weight based moves.
Weak Armor Raises Speed but lowers Defense by one level each upon being hit by an attack. Lowering one of your stats upon each attack you take could be devastating.Anytime you lower a stat , you are weakened and that could lose you a battle. Can be useful though
Illuminate raises the chance you run into a wild Pokemon. It has no effect in battle,has a negative effect outside of battle and is especially annoying in areas such as caves or seas where wild Pokemon are already running rampant.
A Pokemon with Klutz is unable to hold items. This defeats a lot of the strategy of Pokemon. A Pokemon with Klutz can't raise his stats or heal himself using held items.The Pokemon with Klutz will be severely limited in Competitive Battling due to this.
Normalize makes all of a Pokemon's attacks automatically Normal type. Like Klutz,a Pokemon with this ability is severely hindered in battle as Normal moves aren't super effective against any type and don't affect Ghost type Pokemon. This ability is useful in tiers where all Pokemon can have all abilities. In a tier like that you can run a ghost type with skill swap and normalize and block to set up a sweep. Because with block they cant switch out and if the foe's pokémon has normalize its normal attacks cant hit you since you are a ghost.
Stall makes your Pokemon always move last. This shouldn't even need any explanation, this ability makes sweepers impossible and means you can be taken out quickly.
Run Away makes it easier for you to flee from your opponent. This is useless as you cannot flee from a Competitive Battle.
Slow Start temporarily cuts Attack and Speed Stats in half for the first five turns of a Pokemon Battle. This means that for five turns you are in a severly weakened position. Your opponent could use your Slow Start to quickly knock you off. This ability is kinda made to make regigigas weaker.
A Pokemon with the Truant ability can only attack every other turn. This is pretty much like giving your opponent a free shot at killing you. Your opponent can attack twice in a row ! This is a crippling ability you shouldn't want any of your Pokemon to have. This abillity is made to make Slaking weaker.
A Pokemon with the Defeatist ability has its Attack and Special Attacked halved when its health is below 50%. Usually an ability that lowers stats usually raises another. However, Defeatist not only doesn't raise a stat but lowers two stats. If you have a Pokemon with Defeatist, you are pretty much done when your health is below 50%.