Another issue I remember was rosters of large numbers (somebody was using a roster with > 1500 items - I have no idea why). Performance was shot in this instance - Flash gives a "Script is running slow" error, because the code loops through all 1500 items.
I'm not sure how many people will hit this edge case, but another thing that might need addressing down the road. The guy fixed it by tweaking his server to send the roster in chunks.
REINVESTIGATE THIS AS2 ISSUE: With data binding and new AS3 features and AVM2 performance, this should be easily fixed.
Tested with 4000 users in roster.
AS3 XIFF handles it fine.
Rendering the users can take a bit of time, but Flash did not timeout.