What are the main features of a gudwara?

Not sure as to how to explain it but not talking about the shape of the building itself, a gurudwara normally consists of a Langar (dining area), the main hall, and several other halls which are used for more family oriented prayers. Depending on where you are, a gurudwara can quite often have living areas for the "priests" (we don't call them that :P).

If you're in India and are visiting the Golden Temple then you can also go to the pool where you take a dip in the water.