I also take care of most of the business, promotional, "everything else" issues. I would say part of my job is being the coder's lackey, if my partner needs php libraries installed, I'll ticket our server admin. If he needs a feature tested and broken in 5 browsers, that'll be me. I suppose my job is to help make it easier for him to put out code at a higher productivity rate.
A challenge I would say is not having anything to do while he hacks away, which is a lot of the time for early startups. Can't really start promoting or start new initiatives, but theres always something to do, its just a matter of figuring out what.
The challenge was primarily that I had an underperforming technical/engineering team until we hired a professional, experienced VP of Engineering last year (which has made a huge difference).
My advice - get a technical co-founder or early CTO/VP Eng that is immensely technical and totally dedicated. Many on here have described the weakness of having great code and great software with no marketing channel - the other side is just as painful.
I co-founded with an engineer, so it was always a clear cut - I did sales, marketing, finance, he did engineering, we both did product.
No huge issues, I rely on engineering, but I now rely on sales and marketing, and all the other functions.
I considered learning to code at the start, but really you need divide the tasks and stick with what you are good at.
For my latest startup, I teamed up with a programmer. It is super-tough to have a software app without a technical co-founder.