many !!!  but volume is not one of them. eg.  if you have 400(0) dl on one vacation image. and one dl with 400 images. which  do you think will get you more downloads in the future regularly ? your bottom-line is the same $x for 400(0) dl. but one will cannibalize your portfolio and you have to keep feeding the beast like some people. others do better like some people i know who get the same amount of downloads and money for far less uploading.
keywords is definitely the first thing, as it is what gets you found by the clients.
colour scheme too, i find. if you look at many of the images you see on the web pages (with Getty, SS, etc as a side line) you can see a certain colour scheme, and a certain lighting and lighting balance level
that are favoured by the buyers.
and you can also either be one of the many copycats of the top sellers, or be in a niche
where not many fill.  the former does not make you a top seller either.
as a buyer, why would i change to look at a copycat when i already have my bookmark on the top sellers?  some disagree, like always, i too disagree with many things i said earlier 

it all depends on your situation. the best thing to do is find what you do best.
no one else can replace that. and when those certain images sell for you,
make more of those. and hopefully you will find your niche.

or you can be a one stop shop for every thing under the sun, until you run out of ideas.
only you know where to draw the line.
but of course, technique, exposure , etc must be there on every image. the less post-processing the better, as your images will be cleaner , as will your workflow faster as soon as you master that.

still i expected you already mastered your equipment before even thinking of making money with your camera