Having owned 2 or 3 fairly basic to middle of the range models now, my thoughts are in no specific order:
1, Cord length, this may be more of an issue with larger pools, I have a 20x40 ft pool and some robots only have cords long enough for 36-38 ft pools.
2, Quality of cleaning, my Aquabot Turbo T does a much better job of cleaning than my old Dolphin Diagnositc's, it moves much faster too, I attribute much of the better cleaning action to the cleaning jets.
3, Ease and cost of replacing treads, they wear out and on some designs are MUCH easier to replace than others. This is particularly important if you are a DIY type.
4, An adjustable timer is nice, my Dolphins had fixed run time only which worked most of the time, however my Aquabot has a variable digital timer as well as a continuous run option and sometimes it is nice to be able to increase or decrease the run time to do either a basic cleaning or a much more thorough one.
5, I have yet to buy a robot with remote control, but if I did I would want one that had full steering, not just back up and go forward.
6, My pool does not have many obstacles, but if it did I can see the potential benefit in a dual track robot with better steering
7, like others have said, water line cleaning, this really should be near the top of the list, but as I said it is in no particular order.
8, This is one I WISH mine had, but don't know how well it works in reality, and that is a swivel cord to prevent tangling
9, Ease of filter cleaning, and filter capacity
Ike
p.s. there are probably little features I also find I like, like the easy plug computer cord style connector on the Aquabot vs the screw in pin cable on the Dolphin, as unhooking it to untangle the cord is much easier.