Aftermath: Your Physical Life orb sweeper now can only pull off 1 more move!
Arena-trap: You are stuck fighting a Pokemon your weak to/Perish song is activated
Bad-dreams: Losing Hp and asleep
Compound-eyes: When paired with a status move it is quite annoying
Contrary: Serperior+Leaf-storm...enough said
Cursed-body: And I was running a Choice Item too (O.O)
Effect-spore: Yes I finally took down his Breloom...Oh NO!
Flame-body: Burns your Physical Pokemon
Flash-fire: Eat this Ferrothorn! Switches to Typhlosion with Eruption ohh pooh.
Guts: Makes them benefit from the things that should cripple them
Huge-power: STAB Aqua-jet coming from (boosted to) over 400 Attack stat.
Hydration: Rest+Manaphy/Vaporeon..enough said
Illusion: Oh it is a Wobbuefuet I am safe to stat up as much as I want :D uh oh(I actually love this ability though)
Imposter: Ditto is very hard to counter with this
Intimidate: Ruins your Physical sweeper
Iron-barbs: Ferrothorn+Rocky helmet makes him a pain to deal with.
Levitate: It depends on which Pokemon has it, it makes your T-spikes and Spikes useless. And it switches in freely on your mighty Earthquakes.
Lightning-rod: If you use Thunder-wave a lot then you will understand.
Magic-bounce: I WAS SUPPOSED TO CRIPPLE YOU!
Magic-gaurd: You can not toxic-stall them.
Magnet-pull: Pretty much a steel type Arena trap
Mold-breaker: Gengar is now weak to ground type moves.
Moody: Reminds me of my sisters XD but seriously the constant stat changes are a pain.
Motor-drive: See lightning rod
Mummy: My ability is gone
Natural-cure: My beautiful Toxic
Poison inducing abilities: Obviously
Serene Grace: Pair it with things like Iron head and air slash it is insanely annoying
Shadow-tag: Arena trap
Simple: Doubles the effects of Stat up moves (O.O)
Skill-link: Cloyster (O.O)
Speed-boost: Boosting Speed
Static: Paralyzes your sweepers
Synchronise: Spread the status
Volt-absorb: Healing when hit by a move
Water-absorb: Same kind of thing as Volt-absorb
Wonder-guard: Obvious one.
Weather making Abilities: Weather can be pretty annoying if you are not prepared
Weather Speed or Evasion boosting moves: Pokemon that you should out-speed you can not and you keep missing
Too see a complete list of abilities http://pokemondb.net/ability
If I had to choose 4 they would be: Moody, Prankster, Imposter, and Serene-grace