What to look for in a kitchen scale
America's Test Kitchen did a segment on digital kitchen scales. Their judging criteria were:


Accuracy: Does the scale display the correct weight?
Button usability: Are the buttons in an accessible location? Can they be pressed without too much force?
Display visibility: Can you see the display when a large bowl is on the scale? Are the numbers large enough to read clearly?
Easy to clean
Stability: When you push the buttons, does the scale move?


To answer your specific questions:


None of the 10 scales they tested were waterproof - it seems like that may be a rare feature.
If you can detach the weighing platform from the rest of the scale, it will be easy to clean.
They did not have problems with the tare button biasing the weight.
A weighing platform separate from the main machine makes it easier to see the display and press the buttons, even when a large bowl is on the scale.