I am very new to these forums and to SL as well and it may be that this topic has already been thoroughly, discussed, but I'd like to toss this in anyway, based on a couple months experience in world now.
The single feature I would most like to see in the renderer (i.e. client-side software) is support for multiple, separate windows.
That way, using multiple monitors, one could have, say, the Communicate and Inventory windows open on one monitor and the main renderer window on another. I cannot think of any other single feature that would more dramatically improve the user experience.
FWIW, folks here might be interested to check out a recent column of mine at:
http://reddevnews.com/columns/article.aspx?editorialsid=2416
There will be another appearing shortly, also about SL.
All the best,
Arifi Saeed, Research Director, Second Life Office
Canopus Research Inc.
[email]arifi.saeed@canopusresearch.com[/email]
http://www.canopusresearch.com