David Kastrup responds:

"reliable" means that using it does not crash the Windows port.  Further 
features of the XEmacs model I remember were: 
 
a) it pops up with a noticeable time delay (likely X overhead for frame 
creation) during which XEmacs is non-responsive 
b) it steals the keyboard focus on mouse-over-frame window managers if 
you have bad luck (move the mouse cursor at the wrong time/the wrong 
direction) 
c) it comes with frame decorations 
 
Please note that Emacs tooltips are _not_, I repeat _not_ optional.  Why 
doesn't one get a protest storm from users?  Because they don't 
interfere with normal operation.  They are undecorated, they disappear 
as soon as the mouse moves again, they cause no delays and never steal 
the keyboard focus, they never obstruct areas where mouse or text cursor 
are for any amount of time. 
 
All of Emacs' standard menu entries have tooltips (I don't even think 
that XEmacs supports menu entries with tooltips), and pretty much 
everything in mode lines.