Launchers should NOT be put on sd card. Widgets will not run from sd card, but the app associated with it will, like Shazam and Soundhound. I have them on sd card and don't use widgets.
Same for all Media apps. They are fine on sd card. Social apps such as Facebook and Twitter (and Twitter apps such as Plume and Tweetcaster) all run fine from card, but no widgets. (If you like the scrolling updates, keep app on phone).
Maps are fine on sd card. Games also. YouTube/Get Glue/Four Square all are fine from sd card. External SMS /mms apps (Hancent/Chomp....) are fine on sd card, as are mail apps (K9/Touchdown/Kaiten....)
Even certain system apps are fine on sd card, provided you are rooted and can move them. I have Gmail and maps on the card.
Dialers can work from the card. Some people (rooted) have removed the system dialer and contact apps and tried to force other dialers as system apps. This can be done with launchers, but NOT dialers. All dialers rely on info from "native" dialer to work.
If you have questions, let us know! Hope some of this helped!
Sent from my DROID using DroidForums