Hjorimir said:
Rules don't create a great campaign...but they can ruin one.
You hit the nail right on the head. Although unusually clever rules might actually enhance the game by bringing a new element into it (e.g. CoC's sanity rules) mostly game mechanics issues act on a campaign in a negative way. No matter how good the rules are, it is no guarentee of a good game experience... but bad rules can definitely spoil a game.
Could you play with a very "light" rules system? Sure, take a look at Over the Edge's very simple mechanics. Can you even skip on using dice? Sure, look at Amber. Can you skip on having rules altogether? Yes, but then you aren't playing a roleplaying
game anymore since games by definition have rules. You are simply roleplaying, which is pretty much improv without an audience. Nothing wrong with that but it is a slightly different activity.
Rules are light the superstructure of a building. They help give it shape and keep it standing upright. Creativity and storytelling are what does all the finishing and decorating in the building.... they are what make it attractive and enjoyable. However, no matter how pretty your walls are, if the walls of your house fall over you won't like living there.
If you find the d20 rules set too heavy, look at the other RPGs on the market. Most RPGs have simpler rules than d20. Large, well-established games will often have more elaborate rules since they have had time to evolve them. Smaller press games tend to be simpler but there is a lot of variability among them.
I find it best to approach game mechanics as if it were a toolbox. Pick the right set of tools for the job you want to do and it will all go much better. Pick lousy tools or the wrong tools and it will irritate you to no end.