1. Hydraulic lift.
2. Smoothness of glide.
3. Stitch Regulator (with a cruise control)
1& 2 have to do with saving your body. Machine quilting is very physical. I can take a toll on you. I wouldn't do without the stitch regulator either, but if I had to give up something, that would be it.
As for the cruise control, check it out in the A1 booth. It does make a difference.
Roben is right about service. On a longarm, you do most of your own maintenance. It's not that hard, and there's lots of help from your dealer and other longarm users.
Have fun in your search, and PM me if you have questions about the A1!! (Not affiliated, just a really happy customer!)