What should I keep in mind while developing a game for Steam?

Question

I recently did the programming on a game that was just released and i've decided that for my next project I want to do something a little bigger scale. I plan to develop a game to be sold on Steam, it will be 3D. I've done large-scale games in Flash and a small game in C++ written with OpenGL. I realize that flash-based games can be submitted to Steam but I'd like for it to be written in some variation of C. I'm looking for information on a couple things.

1. What engine should I use? Ideally I'd like it to work on both Mac and PC. Seeing as it would be released on Steam, I'd like to use the Source Engine but I'm worried about having to pay licensing fees when I don't plan to make loads of money on it - it's mainly for the experience and as a resume builder.
2. What sorts of things should I keep in mind when developing so that when, and if, it gets accepted, the process of adding the Steam-specific functionality will be less painful.

Answer 1

I don't think it matters which engine you use; from what I've seen, steam accepts games that are of high enough quality (polish) and are relevant to the steam customer base.

Winning IGF or some such also helps.

Answer 2

Unity is a tool I often recommend. You can code it in C#; that's a long way from C, but it's in the same family. It's cross-platform and easy for people with as much 3D experience as you have.