From a wheel builder's perspective, better quality rims appear to be more consistent geometrically and require less fiddling to get true after being laced. I built some early WTB rims ('Power Beam') in which every nipple sat a little differently than the others, and too a lot of work to get perfect even after all the spokes were evenly tensioned. I have also built wheels with Mavic rims and DT spokes where I laced them up and tightened all the nipples to a similar spot WRT their spoke's threads (by eye) and once I had tensioned the spokes they required very little effort to true or straighten.
Another aspect of rim quality is the quality of the alloy. I don't know if most of them are supposed to be nominally the same alloy, but there are definite differences in rim rigidity and also resistance to wear from brake pads. Synchros, in a previous incarnation of the company, made some supposedly good quality rims that normal brake pads shredded the braking surface in a few rides.
Lastly there are features that are generally only included on more expensive rims, like double spoke eyelets, welded seams, and machined brake tracks. Also the cheapet rims are generally a hollow channel ('single wall') while better quality rims have a box section construction (double wall)