You'll need one (and an accompanying app) to get started, and although you'll be able to display the time while disconnected (in Airplane Mode, for example), software updates, feature additions and notifications all require a Bluetooth connection.
The light weight comes thanks, in part, to the fact that the company didn't beef up the battery, instead offering up some software bumps it says compensate for the additional battery drain that comes with things like a revved-up display.