Previously you could only have 40 debuffs up on a single mob. That might seem like a lot, but when you got in a raid with 25 other people all putting up their own set of debuffs and dots, etc... things tended to get a little full.
Daelo, the Lead Encounter Designer, announced today that the debuff limit has been removed*. This is a subtle yet important change for many raiding guilds.
He notes that the default UI won't normally be able to show all the debuffs, but that's just a bug in the UI. The debuffs are still there and working. We don't have any verifiable information yet as to if custom mods can display an infinite number of debuffs.