Ah, the Imperial... it's multi-leveled dive-ness helps it somehow morph into being more than just, well, a good ol' dive bar. As others have mentioned, each section basically caters to a different subsection of patron: regulars and drunkards on the main; hipsters, students, regulars, and drunkards on the second; and on the patio, we find the lost and confused, hipsters, students, regulars, drunkards, and perhaps one or two holdovers from the night before who managed to hide amidst the refuse and sleep there overnight.
So as others have mentioned: couches, popcorn, inexplicable books that I doubt anyone's read since 1987, a menu that will strike fear and heartburn into the bravest of diners, and attentive staff that keep the beer flowing freely.
Can't say much for the draft beer list as it's not very broad, but they do usually have one or two good microbrews available (for a price). Last visit we enjoyed MacAuslan Oatmeal Stout at $7.20 a pint... it tasted a bit off, however, so perhaps they're not as meticulous as they could be in cleaning their taps.
Anyway, if you're looking for a place to chill, have a pint, and aren't at all hungry, there are many worse choices in the area but not many better ones. And its patio is actually pretty cool, even if it does remind me of that time I didn't spend in Milhaven penitentiary.