Android SDK version
A lot of 3rd party apps were developed using an OLD version of the SDK (beta versions).
The G1/Dream runs the final version 1.0 RC19. If the app you are trying to install is not programmed using said android version, it will NOT install.
Only way to install those apps is to download the source code to the apps and update it to use the new SDK instead of the old one (not for non-programmers).
I have already done so with the file browser and got it to work on the G1. However, some functionality has been removed from the SDK in version 1.0 because of security reasons and a lot of apps that were developed using Beta SDK's will NOT be able to be upgraded to 1.0 SDK.