If its gonna be a generic D&D clone, your setting yourself up for a headache. I haven't read the 4th edition yet, but 3rd edition D&D was a book keeping and dice rolling Nightmare. Most other systems were not a lot better...
3 Tips that'll help simplify design, and possibly make it easier to play.
- Keep dice rolls to a minimum.
- Damage is fixed in this game, use that to your advantage.
- The initiative system has to be simple, or better yet, rolled into another mechanic.