I´ll explain everything in the video:

http://youtu.be/FvwTpDU6tmI

Mod edit: Added description of above link:

The problem mentioned in the video also affects normal slime spawning in the overworld. To explain it short: In the latest snapshots the algorithm for slime spawning does not check if medium or big slimes collide with blocks, therefore they can spawn within blocks. This causes the following problems:

• small slimes fill up the Mobcap
• less Zombiepigmen/WitherSkeletons/Blazes are spawning
• farming skulls/nuggets/rods will become a lot harder
• the Nether will become a lot easier since those mobs will spawn very rarely
• a lot of gold farms and ALL WitherSkeleton farms will break in 1.8

Update:

After a bit more testing it looks like the new spawning works this way:

• If a spawnable block was found the game checks if there is something with a hitbox directly above this spot (any block solid or transparent)
• If that is the case no slime is able to spawn
• If there is no hitbox above this single block a slime can spawn in all sizes
• Problem is that there is no seperate check for surrounding blocks if its a medium or big slime
  --> Slimes spawn in blocks and can suffocate

They basically use the normal 2 high mobspawning rules like Creeper, Zombies etc. which makes no sense for slimes since they have completely different hotboxes.