I'd also like to see syntax highlighting in mIRC. It wouldn't make a noticeable difference in performance either, and like Gon said, it could be turned on and off.
What would the syntax be coloured like though? There's only a few types of constructs in mirc. %vars would be one colour, $identifiers another, and probably the brackets for identifiers would be coloured differently.. and commands. This wouldn't really be that helpful since all of that stuff is obvious and stands out. Syntax colouring is generally more useful in a language with quoted strings and different data types etc. One thing that might be kind of useful is colouring mirc commands one colour, user commands (aliases) another, and then unknown commands another.
None the less, it would be a nice aesthetic feature.
