Froyo has JIT compiler - programmer jargon. Ignore this if you don't understand it.
Flash 10.1 support - Really important if you want to watch videos on websites that are not hosted by youtube. Facebook video for example.
support for usb tethering - Some phones can do this before 2.2
improved market - I don't think you need to upgrade to 2.2 for that. It's OTA.
better launcher - Depends on your phone. HTC, Samsung, LG, etc. use their own launchers. So Froyo would not necessarily bring a better launcher.
and you can download all the apps that are only compatible with 2.2, to name a few things - which apps might they be? I have heard of apps that don't work on Eclair, but not apps that cannot be downloaded on Eclair.