When your eyes are "burning" and feel bloodshot, I often tear up and get the sniffles after huge weed doses. Significantly lowered cognitive ability, low attention span, difficulty acting or speaking seriously, accompanied by heavy sedation and laziness (couch lock).
There is a certain sensation which, for lack of better words, feels like being "baked". You feel crispy, a little burnt. If you smoke enough without tolerance, that feeling can persist for 12-16+ hours.... which is what I believe most people are referring to when they say weed has a "hangover". (I find this feeling to be neutral, it's neither pleasant nor unpleasant, and makes me really enjoy cigs in the morning.
In a way, I look at it as the same thing as becoming "drunk" on alcohol. People's behaviors become obviously different than lower doses. They are obviously impaired.