2GB of music storage will equate to around 200 - 600 songs depending on file size, enough to hold your favorites. You can play music without being tethered to the iPhone.
The 75MB limit on photos is fairly small; however, the Watch will reportedly resize images when imported so they take much less room. A rough estimate is that you will be able to store about 100 photos on the device.
The remaining 6GB will be used to hold the operating system, application data, and eventually application binaries.
The Apple Watch will be available to pre-order starting April 10th. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.