Garchomp - Garchomp may not be an issue out of sand, but it sure is what's breaking it. Sand Veil is the real issue here, but Sand Veil dosent break Dugtrio, Sandslash or even Gliscor. Sand Veil breaks only Garchomp, meaning Garchomp is the issue.
Basically, Garchomp generally abuses Subsitute until Sand Veil kicks in, and then Sword Dances, and sweeps a large portion of the other team. Sand Veil and continue to kick in as well, driving the nail in firther and further every time.
As a side note, I feel Garchomp's banning may balance Sand Stream for the OU metagame, allowing Drought to stay as well [As Sand beats Sun with ease]
Drizzle - Drizzle is overpowered even without Swift Swim. Drizzle turns Tornadus, Thunderus, Rotom-W, Starmie, Dragonite, Gyarados and Latios into even more offensive juggernaughts, by boosting the power of their Water attacks, or allowing them 100% accurate Thunders/Hurricanes. Many of these attackers already have obscene attack stats, and either speed, bulk, or the ability to set up even further, making many of them uncounterable.
The number of times I've swept with a BulkyTaunt DD Gyarados in the rain is stupid, so don't write him off. And I don't play OU primarily.
Let's not even get onto Ferrothorn.
Thunderus - Rain or no rain, this thing is stupid. You can't wall it with something like Blissey due to Priority Taunt. It has Zapdos' typing, which is a godsend, as Electric's one weakness is written off, so it only provides resistances and an excellent STAB. Thunderus is hardly fragile either, although it's no wall, it's better in bulk than other threatening set-up sweepers like Lucario.
After a nasty plot, basically nothing can stand in Thunderus' way, and with it's typing, and Prankster, it's rather easy to get one off.
Ferrothorn - The rain makes this thing completely broken, but Ferrothorn is broken in it's own right. It has defenses to rival Lugia and Girantina, when you take into account it's excellent typing. Then, consider Iron Barbs, Leech Seed, Protect, and the ability to set up hazards, as well as two STABS which will generally be 120 Base Power.
Ferrothorn also has some degree of unpredictabilty. It can handle certain counters with certain EV spreads, for example, Max/Max Ferrothorn is not OHKO'ed by Azumarill, while it still retains significant special bulk.
Ferrothorn can also be used in tandem with something like Vaporeon, who can easily wish i back to full HP, with Ferrothorn's low HP stat.
Ferrothorn forces you to run a Fighting Type with a strong STAB and high attack, such as Conkeldurr or Machamp, or a Fire type such as Heatran. Otherwise, YOU WILL LOSE. Look at almost any sweeper that was around in 4th Gen, and you'll see that they always use Fire/Fighting as coverage now, because of Ferrothorn. [IE: Garchomp now almost always runs Fire Fang instead of Stone Edge, meaning he loses to things like Zapdos and Gyarados.]
Something so centralising, that walls a massive proportion of the metagame with such ease has no place in OU.