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I tend to think of CCDs and CMOS as simply a grid of holes or wells that we call pixels - that have certain defining characteristics, their:
1. physical size and shape
2. depth
3. readout noise
4. dark current and noise
As to whether more or less is wells is desirable it depends what you want to do and what optics and targets you are shooting. I imagine a trillion wells that could be binned together to a huge extent would be very interesting and versatile - but too expensive!
As said above - most high end gear is diallying back the number of mega pixels nowadays.
Well depth is a critical aspect for many of us - if the well fills it bleds over onto other wells (blooming) - as wells have a certain capacity and the software treats this precision as an integer (not a float - not wanting to give away any secrets of what might be comming)! If the well isn't deep enough or have anti blooming gates you can overload your image if you have a target with high dynamic range and a bright object in it floods into dimmer areas of interest - e.g. M42
Lastly the readout capability of your wells - what Q/E at what temperature range (do they use low or high quality 12, 14 or 16 bit DA convertors) and dark current charectices of the wells at varying temperatures is very important.
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