Why 2:3? Favorite proportions?

I, too, love odd sizes, but I have to stick with conventional ones for my work, as it makes it easier for the purchaser to find frames for it. People are more likely to buy your work if they don't have to do any work after the purchase.

We're pretty much tied to that format as long as its considered a "normal" or "stock" frame size.

On a good note, 1x3 is getting more popular, its pretty easy to find a 12x36 frame now.
as well as square.

Only when we can sell our art/photos for enough money that the frame becomes an insignificant expense will the cost of the frame not matter.

What I try to explain to people is that you can still buy a standard sized frame and mat the photo to fit the frame. You want a mat anyway to give an airspace between the glass and the print. It doesn't always work if you have something truly oddball like a 12x60, but most of the time it's good. I do 99% of my art prints to fit in a 16x20 frame doing this, but rarely is it a true 4x5 format print. I also usually print it using 11x14 paper since I'm matting everything anyway. I used to buy full sheet mat board and cut it myself, but it's just too hard to handle in my space. Now I buy the full sheets (usually 4 or 5 sheets each of mat and foamcore backing) and just have the frame store cut them into 4 pieces. Save time and frustration while making the prints look better.