A friend of mine directed me to this thread on the Elitist Jerks forum, which contains some interesting comments by Tigole on the debuff limit and 25-man cap. A sample:
...the change to raid size was debated and decided a long time ago. So long ago that there was talk about Naxx being a 25 person raid (with the same difficulty and loot). However, this would have been too harsh a change mid-stride in a live game.