I have a friend from Mississippi and I've heard him use this expression sometimes: slicker than snot on a doorknob. What exactly does it mean? (I guess it's something positive but I'm not too sure myself.)
Doorknobs, generally being made of smooth materials such as brass or glass, are somewhat slippery. Snot, also being made of smooth materials, is comparably slippery.
Combine the two and you have quite the traction-less situation.
This is a vivid metaphor known colloquially as a 'redneck expression', like colder than a witch's tit in a brass brassiere or hotter than two rats [redacted] in a wool sock.
For what it's worth, I have always heard it as
slicker than snot on a **glass** doorknob.
I'm inclined to think this expression may be a coinage from Harlan Ellison (sci-fi writer hero of my youth, who moved on to become a successful Hollywood screenwriter). The earliest occurrence of slicker than snot I can find is from his 1972 The other glass teat: further essays of opinion on television. He writes of some particularly abysmal TV shows that they have vanished...
...with no moans of sadness, for they passed through our culture and prime time slicker than snot on a doorknob (or doo-doo through a colander, depending how vomitous you'll allow me to get)
Based on the fact that doo-doo through a colander occurs nowhere else in Google Books, it seems likely to me that Ellison created both expressions there and then, but only the former survived.
Ellison himself is pretty slick with words - among which I'm quite taken with his “The two most common elements in the universe are Hydrogen and stupidity.”
As regards meaning, per Ellison's usage above, it's an alliterative conjunction of (pejorative) slickness with a situation whereby something exceptionally unpleasant turns out to be exactly where you can't avoid coming into contact with it (you have to turn the knob to use the door).
More recent usages often dispense with the "unpleasant" connotation, with the expression being used as a (sometimes even admiring) metaphorical reference to a smooth operator, or simply a literal reference to slipperiness (icy roads, pavements, etc.)
To me, it means that your hand will come off of said doorknob VERY FAST! Based on the idea that if you grab onto a doorknob that has something unknown on it, your first instinct is to let go. But once you realize that said substance is SNOT, your entire mind and body will be completely consumed with the visceral commitmentment to remove your hand!! Perhaps at speed approaching the speed of sound.
And anybody with a whit of imagination can relate, and will unconsciously wince at the thought of sticking their own hand on a similar doorknob.
The connotations of slick in US usage are broader than in British usage. In the USA, slick can mean both physically slippery ("Be careful! The roads are very slick this morning!") and metaphorically slippery ("His answer was too slick for her liking") or metaphorically polished ("She delivered a very slick presentation at the conference"); British usage does not generally imply physical slipperiness.
It is not apparent from the OP's question whether the referent is a person or a physical object.
For me, describing a person as slicker than snot on a doorknob strongly suggests that they are given to sophistry, equivocation, evasiveness or the deceitful manipulation of appearances; they are certainly not to be trusted.
A similarly described physical object (e.g. a road surface or knife handle) is extremely slippery and lacking in grippability, and may also be unpleasant to touch for either physical or psychological reasons.
It's a colourful expression, but a pretty vulgar one. In other words, it's ideal for teenagers who want to piss off their parents.