Don't make me hate you.
Now that Windows Presentation Foundation (the vector monkey formerly known as Avalon) is really starting to excite people (myself included), tools like Expression (Sparkle flavour) are around the corner, and the ability to animate user interfaces is so readily exposed, I offer up my solemn plea to all coders-come-would-be-designers that want to animate their interfaces: Don't. Go. Overboard.
Please, by all means animate something the first time around. Show me what I'm meant to be doing in gliding, graceful steps. Even take a couple of seconds to transition from one state to another. I'm sure it'll help me understand better.
But every time thereafter, if your animations prevent me from clicking on things as quickly as is humanly possible, please, make sure you provide an option to disable them.
Example: I now "get" how the taskbar works, so I turn off window animation, so I have a snappier task switching experience.
Animations can be a lot like jokes. They're often great the first time, only produce a small reaction the next time or two, and with every telling thereafter, your audience is progressively more likely to try to throttle you with a pool cue, or, say, to try to stuff an airline life jacket down your throat and then pull one of the tags (if the jacket does not inflate, attempt manual inflation with the handy tube). Getting the picture here? Animation causes grumpiness.
To reduce it to a nice, simple axiom for designers and coders alike: If the animation in your interface is delaying my use of that interface, your design is costing me time and money.
So please, keep impatient click-click-clicketty people like me in mind, and implement a "Type-A Switch" in any delay-causing animated user interface.