Some lily roots are as thick as your arm, so yes I would classify that as a snag.
Weeds snags? not really, although there can be plenty of grit, shells etc that can make short work of lines with no abrasion resistance.
As for rushes, some stick up above the water away from the main bed which can be on the edge, so yes I would say with the line going around a reed or rush bed it would be classed as a snag, in my mind, I do know I would want a decent line to cope with it.