In general ...
High sensitivity - required to achieve faint DSO details in a reasonable time
Manageable noise - noise that doesn't over power the faint DSO details (either a lower noise sensor or via external capabilities such as onboard noise reduction, onboard stacking, cooling, outboard noise reduction, outboard stacking, etc.)
Gain control - either onboard or via PC, but you want this control.
These days you need to look at the sensor, the camera and the display software as a whole unit to determine if it all together meets your requirements for EAA. Notice I said "your requirements" ... everyone's needs can be quite different. I prefer to observe at exposures well under 60s and without external PC help (video camera with extensive onboard capabilities) ... others are happier with powerful PC help at very short exposures (or much longer exposures to create really nice captured images), etc. etc.
Then there are discussions about the complexity of the camera setup (does it need other external devices, can it be controlled via a PC connection, how much power does it require, etc.) and how complex the software is (one piece or multiple apps, easy to use at the scope, easy to learn, easy to broadcast with, etc.).
There are very few common metrics that allow you to evaluate the first two items on the list. Some metrics appear to be common but each manufacturer often uses their own take on how to do the measurements which tends to skew the numbers in their favour. In short you need to gain quite a bit of experience in order to fully evaluate the bits and pieces. It's IMHO much easier to read up on personal experiences with the complete packages to see how they might fit your particular needs.