Obviously, for a rider to be pro, they need to be good.
They become good through lots of hard work, taking chances and learning from their mistakes. But, this doesn't mean everyone who tries hard and aims high can turn professional and live off the sport.
Natural talent defines how good a rider can be, and someone who doesn't have the talent, will never make it as a pro.

Now, assuming the rider has a stack of natural talent and potental and fulfills this potental (s)he won't get very far if he can't interact with people, but on a smiley face, and sign peoples' hats after crashing out at the World Champs or whatever.

So in a nutshell: They have to have natural talent.
                          Determination to amke the most out of their gift
                          and be an all-round nice guy.

Ben

with regard to andrew taylor i rode with him last year out in america, and that kid is good! he isnt quite 'pro' pro yet but he's well on his way, although the standard of rider that drop in tv have nowadays is quite poor in my honest opinion, but anyway, andrew taylor, good rider, can do it under the pressure of comps or cameras.

and pro, already been said a few times, but someone who can;

perform under pressure
ride consistent
do the big tricks (not just flips, which is one of the easiest tricks)
promote professionally
and keep the crowd entertained
not to mention getting a nice pay packet

Pat

as much as i hate to admit it i used to watch football when i was 11 and particualrly halifax town who were in the vauxhall conference! in football they were called an amateur-professional team as all their players had another job as well to supliment them.

it seems to me that riders like calla_the_one would also come under this classification as they are the hard working middlemen who have the drive to want to be pro but at the same time have to work damn hard to make a living in a separate thing.

they have similar pressures in riding as the top pro's (because they want to keep getting free gear and save themselves a small fortune) and so have to perform well as a result, and also have the pressures of working a 9-5. i think these guys have the hardest time of it.

in short:

amateur: someone who rides for fun

amateur-professionally: gets free gear but has to work separately to make a living

Professional: doesn't need to work separately as get paid but has immense pressure to do well so they don't lose it all!

A guy/gal with guts/no brains/weed.
 

Pro equals someone who has convinced somebody else to pay them a salary to ride.

Simple as that.

The list of actual Pro's in our sport is very short indeed. I know of at least one rider who has made a World Cup Podium this season, yet is by no means a pro. It would appear being Canadian/American/ Australian is a pretty big help...

i dont know if peaty made a $1million from his sponsorship deal with santa cruz but i know hes renowned as being very shrewed with money! was reading a back issue of dirt with an interview with cedric gracia and he reckons the only riders to have made serious money are: brian lopes, shawn palmer, lord god john tomac, peaty and himself!!  not a very big list in the last 15 years!! however i'm sure there are others (greg holbol?spelling? of sram lives on a big ass ranch so he can't have done too bad!!)