For the purpose of this discussion exercise density is defined as the number of separate exercises performed per week. For example, if you're working out four days a week at five exercises per session, your exercise density would be 20.
Not all exercises are created equal in terms of difficulty, so a routine composed of all compound multi-joint free-weight exercises would have a higher density than one of all isolation machine work. In order to express that we'll use a qualifier word.
Low - Routine consists of all machine, isolation, or accessory work. Medium - Routine is a combination of various exercise modalities High - Routine consists of only the most challenging exercises (medium low and medium high are also acceptable if you feel your routine is halfway inbetween those qualities)
So now comes the questions:
1) How do you rate your current routine? (mine is 17 medium)
2) What's your decision making process for deciding on an appropriate density for your goals?
3) What do you feel represents the optimum density for a natural trainee? Considering the tradeoff between maximum rate of progression and the potential of overtraining, at what point is adding additional exercise counter-productive?
I personally don't agree with your rating system, I'd rate a routine by how well it's producing the results you're looking for. So, in that regard mine is 15 high.
My decision making process for deciding on an appropriate density had to do with the fact that I was coming off a high volume and high density (by the definition that you're using here) program that while incredible for strength endurance, just wasn't really getting the job done as far as building mass.
So, I decided to humble myself and try a program that has been proven effective by a large number of trainees, not to mention is much lower in density and volume than what I had been doing. I also decided to do the program without making any changes or adjustments to it whatsoever. So far it's working great and I'm enjoying doing it a lot. I also chose this program because it allows me to still keep a high level of intensity (or "intensiveness" as some might call it) in my training, which I've realized that I really enjoy.
I do agree with Sentoguy's remark that the rating system is not optimal. What I mean is that it's misleading - even fundamentally wrong - to base a trainees progress on the notion of "exercise density" in the terms that you put forth.
Really, 'Total Work Done' is a more meaningful way to quantify a total amount of reps * load. Attempting to chunk exercises into groups (Heavy, Medium, Light) in the manner you are doing isn't very accurate. It could be improved if you had a few scientists and an Electromyograph was on hand next time you were at the gym. The fact is, though, that exercises are pretty specific for strength (and hypertrophy) gains. The concepts of transmutation and transformation are applied in periodization mesocycles for just this reason.
This is wholly dependent on the goals that one has. There are so many directions ones weight training can go: Are you training for strength, speed, or hypertrophy?
Each direction requires focusing on different aspects of muscular development: strength is all about maximum MU activation and rate coding without regard for rate of force development, speed is about maximizing rate of force development in addition to MU activation and rate coding, and hypertrophy is all about increasing the density of myofibrils in muscle tissue and stimulating protein breakdown (and synthesis).
You seem like an intelligent poster, you are obviously thinking deeply about matters this site deals with. The truth is, though, that a lot of the information that authors deal with is regurgitated ad nauseum - and the rationale of authors' posts are never really developed completely because they are trying to sell you their products.
So! I reccomend that you read the Science and Practice of Strength Training by Zatsiorsky immediately. It is definitely a good starting point for anyone with even half a brain.
You have to find what works for you. CT has said that you should know if the program your doing is right for your body within a workout or two. I couldnt agree with that more. Personally, I the approach I take is this
I do 3 chest exercises on three different days
1 Heavy lifting- Flat Bench Press working up to a 6x6 80%1RM day
1 constant tension- flat dumbell flyes sets of 12-10reps