Aim for as high of polyphony number as possible. Most are nightmares. If you want to combine piano, string and organ sounds, for example, you'll want to be able to do this easily. You should not have to be a computer programmer to combine sounds. Note that this feature is only available on keyboards that are higher priced.
Most synthesizers have an organ-like touch.
Some keyboards will now allow you to change sounds, while some synthesizers will let you change some banks of sounds but not other banks. Also, some keyboards have RAM on-board that will actually allow you to load .wav files or sounds that may not even have been created by the manufacturer but that can be used with many different synthesizers. Number of keys. These full size keyboards are obviously bulkier and heavier to tote. As paradoxical as it may seem, less expensive keyboards come with speakers while more expensive ones do not.