Again, these aren't verbatim quotations from Ion and John, as I simply can't write that fast, but the overall statements are accurate representations of their responses.
Are there any encounters Blizzard have had to alter or leave out due to technical constraints?
There was a boss leading up to the Lich King who you had to heal (Valithria Dreamwalker), and that was a huge challenge for their existing technology. If you think about it, Ion explained, up to that point, every healing spell in the game was designed to be cast on a friendly target, that is to say, a player. So the devs were faced with the task of reworking every healing spell in the game. They didn't want players to only be able to use certain spells on her, as that would have been bad, so they redid every healing spell. The technical team changed the game's design so that the boss basically became a raid team member. Ion and John explained that it's all about working out creative ways to implement the designers' ideas.