[Disclaimer: This is all based on my personal experience. I realize that others may view it differently, and this may be very different in other countries]

I regularly use both the ICQ and MSN Messenger IM networks in my daily life. While they’re both pretty much equivalent in features, especially since I don’t use their official clients but rather the multi-protocol Miranda IM client. I have, however, noticed some differences in style and usage between them. Again, this is purely a personal observation:

• People are more likely to use a nickname or alias for their ICQ screen name than their MSN name.
• MSN is frequently used for business communication as well as personal communication. This follows or explains the previous post. When you contact a business acquaintance via IM, you don’t want to be known as HeLLBoY69.
• Despite all that, the MSN Messenger client – especially the newer versions – is less business oriented and more kid/fun oriented – colorful, loud and with all sorts of eyecandy, ‘nudges’ and other abominations. The Windows Messenger client, however, is a no-frills business-friendly alternative. It’s a shame that it’s lacking some basic features that the MSN client has.
• I haven’t used the newer ICQ clients, but MSN has traditionally been seen as a “toy” IM system by long-time ICQ users. Probably because of its eyecandy, because it comes pre-installed with newer versions of Windows (you have to actually go and ACT to have ICQ installed on your system) and because, well, it’s Microsoft. Interesting to note, though, that I’ve yet to see someone put his ICQ number on a business card or professional email signature. It’s more of a friends-only chatting system, rather than a legitimate mode of professional communications.