Here are my views on the Suspects:
Abomasnow - BL
Through Leech Seed and Hail, Abomasnow will chip at an HP of a Pokemon. Take the following scenario:
a) Abomasnow switches in. Perhaps on a Claydol's Earth Power, Lanturn's Surf, etc.
b) Abomasnow uses Substitute, while the opponent switches out to something that has better capabilities in taking on Abomasnow.
c) Abomasnow uses Leech Seed on said Pokemon. Pokemon breaks the Substitute.
d) Abomasnow switches.
At the end of this sequence, the opposing Pokemon will have 75% (only Steelix), 68.75%, 62.5%, 56.25%, or 50% left. This is assuming it started at full health and factoring in Stealth Rock, which I think is fair to consider a common battle condition. Other Leech Seeders are not as potent because a) Abomasnow has a 120 BP STAB SE Blizzard with which to hit Grass-types that are immune to Leech Seed and b) Abomasnow cancels out Leftovers recovery of any Pokemon excluding Ice-types and Clefable.
Now, there are many variables that can nullify some of Abomasnow's power. 50% recovery moves, fast Encores and Taunt, U-Turn, etc. First of all, to force Abomasnow out, the Pokemon in question must be able to outspeed Abomasnow, use a slower U-Turn, Encore Leech Seed, or use Whirlwind, Roar, or Perish Song. If it is slower and does not have U-Turn, Whirlwind, Roar, or Encore at its possession, it will be helpless to an infinte cycle of Substitute/Leech Seed.
So basically, in order to force out Abomasnow, the Pokemon:
a) Has to outrun Abomasnow.
b) Has to threaten to KO (also ie; Poison) him, Taunt, or Encore.
c) Does not take considerable damage from his attacks or Stealth Rock.
d) Has reliable recovery.
or
e) Can use a slow U-Turn and switch to something else.
Satisfies conditions a - d:
Kangaskhan
Miltank
Satisfies conditions b - d:
Clefable
Hypno
Umbreon
Satisfies condition e:
Mespirit
Uxie
If a Pokemon does not satisfy condition a or e (ie; Chansey) it will just be stalled out in a cycle of SubSeeding.
Condition b is self-explanatory and applies to forcing most any Pokemon out (the first part, anyway.) Condition c is the same way as well.
If a Pokemon does not satisfy Condition d, it cannot keep switching back into Abomasnow unless it has excellent Wish support.
Condition e, a slower U-Turn, will result in the breaking of Abomasnow's Substitute and for Leech Seed to be negated. However, both Uxie and Mespirit do not have reliable recovery, so they do have the potential to be worn down.
Support Characteristic
A Pokémon is BL if, in common battle conditions, it can consistently set up a situation in which it makes it substantially easier for other pokemon to sweep.
I believe Abomasnow is BL because it falls under the support category. Abomasnow's stalling abilities can easily create holes in the most defensive of walls. Arcanine's going to have a tougher time switching on Glaceon at 50% from tackling Abomasnow. Abomasnow simply has the resources to just cause opposing Pokemon to disintegrate, 12.5% at a time.
A word on Rest: Yes, Rest does make Abomasnow's job slightly harder if it cannot achieve a 3HKO. But by forcing the opponent to Rest to beat it, Abomasnow still sets up a situation in which it makes it substantially easire for other Pokemon to sweep, thanks to the "random nature" of Sleep Talk, and the fact that the opponent has to give up a moveslot for Sleep Talk in which a significant move might have been used otherwise.
Crobat - UU
Unless I'm mistaken, the argument for which it was nominated was that Crobat can use Taunt to shut down other leads (ie; Froslass/Spikes) and then scout with U-Turn or set up weather and then U - Turn, leaving it with a significant advantage.
First of all, shutting down the non-attacking shenanigans of other leads does not fall under any characteristics. Definitely not offensive. Steelix can still Gyro Ball, Uxie can still Psychic, etc. so Crobat cannot use wall and outstall the majority of the metagame.
Support Characteristic
A Pokémon is BL if, in common battle conditions, it can consistently set up a situation in which it makes it substantially easier for other pokemon to sweep.
Well, it technically can. If Crobat Taunts Registeel, it can't set up Stealth Rock and thusly, Moltres can sweep substantially more easily. And Registeel has to Iron Head, meaning Blaziken gets in for free.
However, that's only one Pokemon. If Uxie were leading, Crobat U-Turning to Drapion could end up with Uxie U-Turning to Steelix. And the Steelix Crobat tries to Taunt could end up using Gyro Ball instead. So really, it's not very consistent. I think this point is already a beaten dead horse, so I will address another issue:
Weather
Crobat's distinguishing characteristics are a combination of speed (and Taunt), defensive ability, and U-Turn. Which are great assets, but not broken.
Think of it this way: Can your lead stop Electrode from using Rain Dance with a faster Taunt, kill Altaria in one hit, and overcome the nature of a U-Turn from a fleeing Uxie...all at the same time? What happens if you face three consecutive players each using a Rain Dance team with these three respective leads? What lead that cannot deal with Crobat can deal with all three of these leads at the same time? Basically, if you can't stop Rain Dance from Crobat, you can't because of only one of its traits, not all three at once.
Staraptor - BL
If Luxray, Rotom, and Ampharos become bog standard overnight, maybe.
Gallade - UU
Gallade has become a bit iffy for me. It was pretty susceptible to revenge kills, mainly from Crobat. Yes, there's not much that can switch in it directly and then kill it from there, but you can pull stuff like switching in Mismagius on the SD and then going to Crobat on the Shadow Sneak. Unlike Staraptor, Gallade isn't as efficient with only two attacks, is slower, and doesn't have Roost, so Substitute leaves some things uncovered. Gallade is a bit more prediction based as I see it.
Raikou - UU
I think the main issue with this is with offensive teams. Speed and Substitute means that the Pokemon coming in will be forced to take a hit or two in breaking Raikou's Substitute. But still, Raikou can be feasibly stopped by a number of Pokemon, including most notably Chansey.
Froslass - UU
I'd love to kick this thing out of UU. But I've found that Spikes isn't that hard to get up even if you're not Froslass. Its other supporting options are pretty predictable now.
Shaymin - UU
Not a suspect yet, but looks like it's on the road to becoming the next one. I think if Crobat returns to UU, it will do a stellar job in keeping it in check.