evilleMonkeigh over at Delta Vector has a great ongoing series on wargaming design. While I don't agree with everything that he says there are a lot of great points, food for thought, and examples of what he believes makes a game great and what makes a game less than stellar. It's a fascinating series and if you haven't already I heartily recommend giving it a look and then thinking about the rules systems you use. It's a great resource for those of us out there who are thinking of writing our own rules, and a wonderful read even if you're just a dabbler in the hobby.
If you're interested, the first piece can be found here.