Do you want to leave it exposed, or with doors? We had a pantry like that in our old house (a little shallower). We used doors to match the kitchen cabinets, and the interior was lower pull-outs and upper shelves. In your case, because of the 20" depth, I'd probably do lower deep shelves and upper shallower shelves, and cover the space with some sort of doors.

It looks fairly deep. What about shelving on the side & back walls?

Edit-ignore my suggestion now that I see dimensions.

Definitely want doors. Just want to cram as much in there as possible...worried it's so shallow.

Not sure if you've seen these already, but just in case not, these threads may be useful

http://ths.gardenweb.com/discussions/2401919/pantry-photos-pics-of-pantries

http://ths.gardenweb.com/discussions/2697558/show-me-your-walk-in-pantry-lighting 

What do you want to store in there? The food for the week and staples? or will it include "stock ups"? Will you use the toaster and microwave and coffee grinder there? Will you store small/medium appliances in there? The use will help guide the outfitting.

The only issue I see with using that space as a pantry is the window. What sort of climate do you live in? What is the window exposure (N/S/E/W)?

If you live where it gets hot, the window may be an issue letting in even more heat than the walls of the house. I recommend seeing if you can include some kind of ventilation. If you live somewhere where it's never hot/cold, then maybe that window could act as your ventilation! I would, however, do something to prevent the sunlight from coming in too much (heat build-up, affect on food, etc.)

See the thread below for some ideas. Please, don't post on it as it's an old thread and most people will ignore it if you do - start your own thread if you have questions this thread and the threads above don't answer (search the Forum for additional pantry threads as well.)

http://ths.gardenweb.com/discussions/2694544/i-have-a-pantry-suggestion-ventilate

The great thing about a shallow pantry is that you can reach to the back easily and so have ready access to what's in there. I'd just fill 'er up with ~18" deep shelves. If once you get stuff in there, you can't readily access what is towards the back I'd add some large lazy susans (see pick, 2nd & bottom shelf) in one of the corners - this has improved access to the shelves we have. Also the rack on the top shelf has been great for vertical space ($20 at the container store).

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As bpathome said, it depends on what you need that space to do. What doesn't fit elsewhere in your kitchen? In general, it's 20" deep, so a bit less than a standard base cabinet, so would probably be most efficiently used as a floor-to-ceiling cabinet with doors. How that cabinet would be arranged and outfitted depends on what you need to keep there.

Random idea: make it a baking center. Have a pull-out counter/workspace at waist level, then upper doors that recess back into the pantry. On the shelves inside you could keep all your baking supplies (flour, sugar, etc.) and your mixer.

Along the same lines, you could set up a coffee station, or a beverage station with a mini fridge below, wine glasses above, etc.

We had initially talked about making it a dry bar or a beverage station, but I don't have a place to store staples/canned goods et al.

I say keep the window. I have one in my pantry and I love it.

If cost is no concern I'd be looking at the vertical pullouts that were posted above.

If cost is a concern, I think I would do shallow shelves, around 4" deep on the sides. They would be deep enough for 1 can, nothing would get lost, everything would be in sight. I'd place these shelves rather close together. I might leave 1 or two of the shelves farther apart, say for ketchup bottles or other tall bottles that you'd want to store. Space the rest of the shelves so that there is just room enough for the height of 1 can on each shelf.

I'd do the shelves in the back somewhere between 12" and 15" deep.

I would not want the shelves to be 20" deep. It's too deep. Things will get lost in the back and you will have trouble retrieving items because things will probably get stored in layers.

Have you posted a floorplan? The creative minds on here might see some other solutions for your space.

I'm not sure about the style you're looking for. Our kitchen had a step in pantry, over a heating duct, in what once was an entry from the outside to the stairs to the second floor (an addition with indoor plumbing, including the kitchen, was put on in the 1910s). Ours was/is not as wide as yours, but deeper. We split the difference. On the stair side, when we get around to it, we'll add a shallow-ish closet (it's currently a bookcase). On the kitchen side, we had 3 drawers built in, with cabinet doors above. It looks like a built in. We went as deep as standard drawer pulls allowed.
I'd consider doing something like that, and leaving the window be. The built in would end at the bottom of the window.

I would also prefer a built-in below the window.

I'd be very hesitant to do any shelves deeper than 12" unless you have one low shelf at 15" for small appliances. The balance of your shelves should be 12" or less - anything deeper than that and all your stuff will be lost forever!

I like a combination of laughablemoment's idea with keeping the window intact.

I have come back time and again to this kitchen pantry from Lee Valley Hardware:

If you need it for food storage, I'd probably do a standard pantry with rollouts, something like this:
pullout pantry (dang houzz won't let me paste image)

Another pantry idea

Another option: It looks like you may have space on the walls on either side to hang barn doors instead of having standard swing-open doors.