Most of the problems I mentioned: conversations of any length could be read in the normal English left-to-right and top-to-bottom direction, adding someone to a conversation already in progress was easy (as was removing someone), I don't think spam could ever have been as big a problem on Wave as it is in email, changing one's mind and the ability to edit what has been said was trivial in Wave, and it created a single, canonical conversation as opposed to the fusillade of conversation chunks firing back and forth (and multiplying times the number of participants) that we've become so familiar with in email.