MSN Explorer is kinda like a skin for IE. There was a brief period of time where I worked as a quality assurance monitor for tech support calls for MSN. Most of these dealt with MSN Explorer. This was a while back and there have been quite a few upgrades (if you can call them that
You can see a learn alot about MSN Explorer by clicking "Help & Settings" (top right) -> "Show All Settings" (left side). The biggest problems I heard while monitoring dealt with "Poptimization". MSN Explorer will build its own dialer, connect to a sign-up server, and retrieve local numbers (if available). Of course, this is if you're using it with MSN internet access. Other issues were too numerous to list here (and I have a hard time remembering them in detail...lol)
Shortcut keys are almost a must, such as:
Ctrl+Shift+S = Display status bar.
Cut/copy/paste shortcuts for email.
F5 for Refresh/Reload.
In older versions, the sidebar couldn't be minimized. The only way to browse w/o it was to hit Ctrl+N to open a new window which didn't include the sidebar.
I dunno, it's attractive at first, but the lack of functionality grows old fast. I still mess with it from time to time, but I prefer Mozilla and IE