Joints used, adjustability in the control arms/track bars, quality of the spring (sagging), included parts, shocks used. Really it depends on the driver and what they want. Generally though some of the less expensive kits just don't have the same quality and longevity of others. There are some that go against that seem to make most of their users happy like Zone though.
I know guys that have been running Old Man Emu for 100s of thousands of miles with no issues. That's why I have OME, under my jeep .. Yep, twice as much money as a Zone, or BDS, but has anyone seen any negative posts about OME? There are other quality lifts out there, but all I want is 2.5"s of lift.. In the world of off road aftermarket products you do get what you pay for..
04' Rubi, with all the gizmos we need for long range excursions, / home modified excursion trailer;
96' Lexus 450, locked, loaded and ready to roll..
'12 VW New Beetle