I am often asked what are the essentials that I need for my craft, especially with all the traveling I do. I've often said that many of the essentials are within, but there are some essentials that are along for the ride. Truthfully I am an old-fashioned photographer who switched to digital directly (and somewhat reluctantly) from Large Format so I still have a "Large Format" mentality. So the first essential for me is the camera itself. You might think this is taken for granted, but your camera is never taken for granted. You need a camera that can produce high quality results for YOU. That means you have to really know your particular camera. It needs to fit in your hand well, not be a burden, and will give you all the range you need in this digital age for your post-capture work. I love the Canon 5D Mark III. I'm a Canon guy because that's the camera that fit my needs from day one of my digital life.
Now for my way of working, the second essential is a sturdy tripod. Sturdy is my first criteria because I've been swept into the ocean in Iceland, been caught in storms, had my luggage end up in another country and return to me looking like an elephant stepped on it. It must also go to at least eye level, which is obviously different for everyone! A sturdy head is important but it can't weigh too much since you are lugging this appendage up cliffs and down canyons. I think most of the carbon fiber tripods fit the bill from Benro to Manfrotto to Gitzo.
The last thing is just the right size camera back pack. Now it needs the right amount of space for the necessities and no more. I've seen people on my tours lugging three cameras, 6 lenses, two tripods and more polarity filters than fill the cases at Hunt's. You need it lightweight and comfortable, easy to get things into and out of, especially if you're doing so in a hail or sand storm (not kidding here!). Tamrac or Lowepro are great and you want them well padded. I have recently invested in a waterproof case for transporting equipment to a location, since I have now twice had a camera end up in the ocean while still inside a camera case. So depending on the places you go and the challenges of the environment, there will be just the right case. And last, don't forget that you need it to fit easily into the overhead compartment on a plane without arguing for 20 minutes with three attendants and pulling out your tape measure while pointing to the written guidelines for carry-ons.
But the most important essential is peace of mind. Without that I find, the image can be quite elusive.