Units that I, a competitive player, avoid at all costs?
Orks.
Eldar, barring Dire Avengers, Fire Dragons, Wave Serpents, Falcons, Fire Prisms, Vypers, Eldrad, Yriel, Farseers, Autarchs, Warp Spiders, Storm Guardians.
Chaos Space Marines, barring Plague Marines, Kharn, Dreadnoughts, Rhinos, Obliterators, Predators, Havocs, Chosen, Raptors.
Calgar, Tigerius, Sicarius, Chapter Masters, SM Honour Guard, SM Vanguard, Thunderfire Cannons, SM Scout Squads without Telion, Chronus, SM Vindicators, Whirlwinds (any flavour,) Land Raiders (any and all, with I suppose the exception of Dark Angels in a 1500 or less game,) SM Devastators, SM Assault Marines, anything in the DA book that isn't Deathwing/Ravenwing/Tacticals/Ezekiel/Azrael/Land Raider Crusaders/Razorbacks/Rhinos/Predators/Venerable Dreadnoughts, Ironclad Dreadnoughts, Furioso Dreadnoughts inc Librarians, Death Company, Death Company Dreadnoughts, SM Terminator Squad (not Terminator Assault!) Eversor Assassins, Brotherhood Champions, GK Brother Captains, Mordrak and Ghost Knights, Lycheguard, Triarch Praetorians, Monoliths, Ethereals, Vespid, Hammerheads with Ion Cannons, Sniper Drones, Drone Squadrons, Kroot Ox, Lictors, Pyrovores, Parasite of Mortrex, Doom of Malan'Tai, Rippper Swarms, Necron Special Characters bar Orikan and Anrakyr, arguably Imotekh I grant, Mawlocs, Biovores, Pyrovores, Rough Riders, Ogyrns, Techpriest Enginseers, Basilisks, Lema Russ Vanquishers, Chenkov, Stealth Suits, ...
Fuck it, my arm hurts after that. Suffice it to say, we're about half-way there.