You can look at it from perspective of the sponge, or the topsheet.
I crave the maximum sponge that will result in 4.0 mm total thickness. Sponge is where you get power and spin, which drives your control.
I value a topsheet that will wrap the ball and allow the sponge to use its energy. This means the topsheet must be elastic, very elastic. Also, it must be durable. Counter-looping strong topsin is a severe stress to a topsheet and not many modern topsheets are totally immune to a small tear or tears in the rubber. Especially the older speed glue jobs... they simply weakened the topsheet at the expense of short term awesome performance.
Having said that, I like the modern medium and somewhat thin topsheets as they get the job done the best. I still like some thick topsheets like 999 or Dawei Inspirit. They are immortal and almost bullet-proof.