There are so many things professional pianists look for, but that subject will remain for series of separate blogs. For an average family purchasing an affordable piano, the piano should provide three things: a high level of responsiveness, a good tone, and good stability.
For most beginners, a piano should have keys that are even in weight. There should little to no play before the hammer travels once the key is depressed. When a key is lifted, the sound should be damped immediately. Double repetition and aftertouch is not as important for a beginner, but a grand piano action will always be superior.
Good sound is different from excellent sound. Long sustain, even voicing, and clarity are the hallmarks of a good sound. When a key is depressed and held down, a piano with sound will sustain for more than 10 seconds. Even voicing means that most of the keys will have a similar quality of sound. Nobody enjoys playing a piano with lots of randomly harsh or mellow notes scattered through the keyboard. Clarity means that you can hear what is going on without the sound becoming muddled together.
Lastly, stability includes how long the instrument stays in tune (6 months minimum), how long the piano will last (20 years minimum), and how durable the piano will be (normal wear and tear).
Every piano in our store is a suitable beginner piano. We have the best technicians in the Southwest works extensively on every piano across all price points to ensure that these fundamentals are covered.