Hmmm really good question.
When I bought my really good machine - it contained more features than I really needed. Since that time, my use of it has changed (from sewing clothes, into quilting) and I have used more of those features now. (Even today - 20 years on - I still don't use all it's features).
If I was buying again I would sit down and work out what exactly I wanted the machine to do (I'd want dual purpose). So things like; straight stitch, zig-zag, memory button hole, stitch regulator, walking foot, 1/4" foot, etc. Then I would check out what modules are out there that would suit my requirements.
I prefer the more "manual" type models rather than computerized. No logical reason - they just seem more stable.