There's a problem with synchronous internet communications, like IM and VoIP. The problem is that I am not always in one place, looking at one screen, on one network and connected to one profile. My PC at work stays switched on and connected to the internet all week, but I'm not sitting there all week. I go home at night, or I go out at lunchtime. During those times, my home computer or my phone might be a better way to reach me. Plus, I am listed on four (!) individual accounts: Facebook, MSN, Google Talk and Skype. The best I can do is use Pidgin for three of them, but Skype requires its own client. All of those profiles need to follow me if I step away from the computer. Lastly, I have some contacts that are on multiple profiles, but it's tricky to group them together into one record (impossible unless I use a cross-protocol client like Pidgin). If I want to contact someone, which program do I turn to, on which machine? It's all separated a bit too far from "me in contact with my friends, family and co-workers".
Mokalus of Borg
PS - Apps and operating systems need to be invisible.
PPS - I often feel like this topic needs its own label on this blog.