What/Who decides when a weed is a weed?
Just to dispel the assumptions of all the curious dopers out there, this is not about pot. 
This is about horticulture. What is the determining factor(s) that categorizes a plant into the poor “weed barrel”?
Most of them are quite complex and beautiful…
It’s purely situational. A weed is any plant growing in a place where it is not wanted. A rose bush is a weed in a cornfield. A corn stalk is a weed in a rose garden. In the depths of wilderness, there are no weeds.
Do you want it there?
No = Weed
Or alternately, they’re all weeds.
Personally, most things that others call weeds, I call wildflowers. Except for thistles; they can all just wither and die.
I have patch of chickweed growing in my back yard with my lamium that is green and lovely - totally a weed, but it’s doing something good there, so I’m leaving it alone. I also have some weeds that make a dandelion-like seed head in the front that I’m leaving alone, because they’re the only green things that will grow in that area (under a big spruce). I’m actually thinking of adding clover to my front lawn, because it stays nice and green when my grass turns brown in the heat and drought of late summer. I seem to be re-thinking my weed ideas. 
But thistles can all wither and die.
except for artichokes, right?
I loves me some artichokes!
http://www.whatbird.com/browse/objs/All/birds_na_147/141/Eats/12323/Thistle/default.aspx
Rabbits eat the plant.
"Origin of botanical name
The botanical name is derived from the Greek words onos (donkey), perdo (to consume), and acanthos (thorn), meaning ‘thorny plant eaten by donkeys’."
Apparently there is a kind of weevil that feeds on Canadian thistle and you can buy it to release. Bugs that eat Canada Thistle
It’s a weed when it’s not supposed to be there, because you or other people don’t want it there.
This is incorrect. Plants that are not natural to the area are called an invasive species, aka, a weed.
My wife would literally rather have bare dirt in her flower beds than plants that she doesn’t want to be there; with the result that half her flower beds are now sun-baked hardpack. IOW, she has the same attitude towards weeds that Communists have to free enterprise. 
Invasive species generally start their invasion as the exact opposite of “weeds,” in that they are deliberately imported to the new region by people. They are planted in gardens or near human settlements, not in the wilderness.
Coincidentally, I was talking with some researchers at one of the Big 5 seed corn producers yesterday. I asked this exact same question when one of them say “weedy” to describe a plant. They clarified that weeds had two characteristics:
This differentiates them from crops where you do not want the seeds to spread at all as this would mean less yield/less money. This is one of the first steps of domestication and can be applied to wheat, corn, apples, even flowers like tulips and poppies.
They even said that some trees could be classified as domesticated now as they have been selected for fewer seeds and not as successful at spreading.
I thought it was really a good way to put it.
True in many cases, but many are also accidently imported, not deliberately. Often as contaminants mixed in with desirable crop species. For instance, possibly the most widespread invasive plant in California is yellow starthistle, which has spread to 41 states, according to that. It was probably a contaminant in alfalfa seed imported from Chile (where it wasn’t native, either). Iconic as it is, tumbleweed is not a native species to North America - it is believed to have found it’s way from Russia or the Ukraine in flax seed. Quackgrass and knapweed were probably also contaminants in agricultural seed.
IIRC, the classic definition is something unwanted that has some combination of characteristics such as thorniness, irritable to the skin, spreads fast/invasive, chokes out other plants. Sort of the vegetative equivalent of the term “vermin” I think.
National parks are highly subject to the accidental importation of invasive plants, particularly dandelions, whose seeds come in on car tires and shoe treads. There is an active effort in Denali, for example, to eradicate dandelions in the park.
We have mint growing in our garden, lots of it. In one particular corner of the garden it is a plant, in the rest of the garden it is a weed.
Yes, many years ago the kid in the house behind mine decided it would be nice to have morning glories on the fence between us. Now they have taken over every yard in the block, choking everything they encounter, even climbing up telephone poles and across the wires. No amount of week killers or yanking them out can stop them.
And still, you can go into any garden store and buy a packet of morning glory seeds.
Turn and run!
Nothing can stop them,
Around every river and canal their power is growing.
Stamp them out!
We must destroy them,
They infiltrate each city with their thick dark warning odour.
Alright, we’re getting a little carried away here.
The definition of weed most gardeners recognize is “plant in the wrong place”.
Invasive plants may be classified as weeds by most, yet in the right climate and circumstance are good garden plants.
Incidentally, most invasive “exotic” plants were not originally planted by gardeners, but were accidental imports/contaminants (purple loosestrife, for instance, got its start in the New World due to seeds present in ships’ ballast).
Old farming adage:
A plant growing where it is not wanted is a weed.
A plant growing where it is wanted is a flower.
A plant growing where it is needed is a crop.
On another board I visit, someone points out in his sig that a weed is a plant that has mastered every survival skill except how to grow in rows.