Having the mouse activate things (like help on what to put in a text field) just by hovering over some spot is pretty neat.  However, with this being used more and more, it’s starting to cross over from neat to annoying.

UI Guideline: If you are going to do a mouse-over help box for a form field, make sure that the pop-up text box containing the help doesn’t pop up over the field in the form!  Seems like an obvious thing, but most of the time the stupid things pop up right over top of where you are typing, and you can’t see what you are typing anymore.  That’s less than useful.  Also, keep the area for the pop-up trigger fairly small.  There are few things more annoying than shoving the mouse out of the way to type, and accidentally ending up over a pop-up trigger which either shows a big old help box right in your way, or even worse, triggers a drop-down for another field which grabs the keyboard and prevents  you typing where you wanted to type.

It’s all about politeness, really.  In short: Don’t get in my way, and don’t make me do things I don’t want to do right now.  You should try to minimize user irritation with user interfaces, and that’s fairly easy to do if you give a little thought to the design.  These days, I’m seeing too many user interfaces peppered with “whizzy features” which are more annoying than not.