Why Syl appears as human to Kaladin, she floats/walk unsupported, sits in a chair stationary in a moving reference frame to kaladin. As leaves she flies around, or a streak she flies.
Pattern travels as raised ridges while 2d, but floats as a fractal ball in 3d.
Wyndle moves only via his growing vines, not sure we have a floating example yet for him.
The Voidspren while appearing as human at eye level had to be supported by, (sprenrock?)
This tells me that the voidsprens' internal logic/rules/etc requires that she be supported by underneath, Not 100% sure of the implications but in basically every other way the voidspren was described as a contemporary of an honorspren with the exception of color and how she manifested.
The fact that she was support by stone, connected to it, when she manifested as human means that stone is very important to the spren. much like wind is to honorspren, vines are to edgedancerspren (cultivation), stone is to void?