Whoop, late reply...
The #1 reason is that I can't stand the layout in chat.
This person says:
this
The other person says:
that
I prefer something more like:
This person: this
The other person: that
I also can't stand the stupidly long names people give themselves. They put entire sentences for names. That's not a name!
PlEaSe DoNt TaLk To Me LiKe ThIs says:
Hi Daniel!
Daniel says:
That is NOT a chatname.
My preference is like in AIM...
DanieLoony: This is a chatname.
Boony LoB: Oh.
I also don't like the smilies and the general layout, although they're more aesthetic reasons. It also doesn't have direct connections like AIM does, and Yahoo! Messenger out-does all the other IM's when it comes to voicechat and webcams. Y!M also has my email, so I get my alerts that way. I haven't changed email since I first got it, so there's no chance of me moving to hotmail (and I do hate hotmail with a passion).
There's also the fact that 99% of the people I know have either Y!M or AIM.
My AIM logs file has just over 300 names that I've chatted to on it and I can use multiple names on it, since I have DeadAIM installed (AIM has a lot of hack-programs for it) and Y!M has over 150 names that I've chatted to.
I don't know if you know about mIRC, though... that's like a chatroom system where you can set up chatrooms for a website that are on fulltime and you have control over them. You can log into them via a java applet on a website or through the mIRC program. The program is highly customisable - you can set it to highlight lines that words are said on, blink when certain words are said (I have it set to blink on all of my nicknames, so nobody misses me - and so I can see what they say about me while I'm idling
), you can use skins and scripts, it's just... very customisable. You can host chatrooms on existing servers or start up your own, and anyone can start up a temp chatroom just by typing a simple command.
Examples of mIRC - Yay for Google image search.
Example of jIRC (java IRC):
http://www.eyesonff.com/chat/
Mind you, that's a really crappy java applet. I really need to tell Cid to get a new applet. =/ Stupid drop-downs... bleh.