The titlebar of the main mIRC window is limited to 153 characters (+ the word "mIRC" at the beginning) which is sometimes annoying when the title get truncated (or when you are displaying your own text) while there's still space available, I would like to see that limit either removed, or extended.
Also, when using /titlebar, mIRC still add its default text after our own text; again it can be annoying if the purpose is to display our own text only, I use $str($chr(160),X) which is fine, but I think it's something we should be able to do without a workaround.

mIRC does not place any limit on the number of characters in the titlebar - the limit is actually imposed by Windows itself. As for the text being displayed after your /titlebar text - you are probably referring to the titlebar text for the currently maximized MDI window? Again, this is an MDI window feature that cannot be turned off and is a standard part of MDI windows. These limitations can be overridden by using a custom display/skinning routine for the titlebar, however this can lead to other titlebar display complications, so it is not really worth going down that path.

Ok, I thought you were already using a custom routine for titlebar.
What would be the eventual complications if I may ask?

mIRC was using a custom titlebar routine in older versions in order to display Unicode text. This is no longer needed in the latest version. The problem with modifying something like the titlebar, which is an integral part of the Windows interface, is that it ends up working in a non-standard way and you have to constantly tweak it to make sure it works correctly. My old custom titlebar routine never quite managed to work as well as I wanted - there was constant flickering during updates and what mIRC displayed in the titlebar did not match what Windows itself displayed in the rest of its user interface, such as the text in the taskbar icons. So I will generally not try to customize something like this unless there is a very good reason to.

Ok, that's understandable

Perhaps most prominently, it seems to make the Aero Glass effect stop working for the mIRC window border (including title bar).