The greatest strength of a digital piano is the depth of its sound and its ability to perform in a nuanced, realistic fashion. As such, it needs high-quality source material and a keyboard that accurately represents the style of its player.
The most important specification for advanced musicians is maximum polyphony, which represents how many unique sounds it can produce at once. Lastly, a digital piano’s practicality is important depending on the needs of each individual. The construction of each instrument will decide how versatile it will be when transferring from a home to a stage environment.
Available Voices: Digital pianos often walk a fine line between featured voices and useful ones. While having a bounty of options is nice, the playability of each depends on their quality and type. For this reason, less is often more. A professional grade digital piano will feature fewer unique voices at a higher quality while lesser models will feature amusing but ultimately useless effects.
Touch Response: When playing an acoustic piano, pressing keys slow and soft will produce a significantly different sound from notes played hard and fast due to string length and tension. Keyboards that are sensitive to the fingering styles of their players are more nuanced and emotive, so the instruments that best replicate the keyboards of acoustic pianos are generally the finest. More at http://tuleburg.com/discussion/129596/inexpensive-options-for-88-weighted-keys.
Maximum Polyphony: The maximum number of notes a keyboard can produce at once is its maximum polyphony. This is important for the piano because players will generally want to play chords and melodies at the same time. For a beginner, it may appear strange to want a polyphony higher than the number of fingers a person has, but advanced and even intermediate players will find that it’s not very hard to play and sustain 64 notes at once.
Digital pianos come with either a 32, 64, 128 or 256 note maximum polyphony.
Portability: Digital pianos are significantly lighter than the acoustic instrument, but stage musicians may desire specific features in order to transfer easily from venue to venue. Portable pianos meet this need most simply, with detachable pedals and other design elements to make them easier to move. However, it is also necessary to balance portability with overall quality.