The above is an interesting video on a study about multi-tasking. The study essentially lends credence to the adage "Jack of all trades; master of none."
This is relevant to muscle building, because building muscle mass is really not that hard. All you have to do is focus your training. If you're looking to gain mass, then keep your training and diet simple.
A lot of trainees try to cram in 3-5 exercises per body part, thinking that you need to train a muscle from different angles. The fact is you do need to train a muscle from multiple angles in order to fully develop it. But you don't have to hit all of those angles in the same workout. If you do, then your nervous system gets confused trying to perform different exercises. You become the jack of all trades, master of none, and your physique reflects the fact you're still at square one with regards to muscle size.
If you're in need of serious muscle mass, then focus your efforts. Don't train like a bodybuilder and do every exercise in the Encyclopedia of Bodybuilding. Train like a powerlifter and focus on getting strong on a few basic compound movements.
The key word is to train like a powerlifter. Powerlifters develop a lot of muscle mass by focusing on heavy weight on a few exercises. Follow this training strategy, but instead of working the traditional power lifts (bench press, deadlift, back squats) choose multi-joint exercises that develop bodybuilding symmetry: pull-ups, incline barbell press, front squats and deadlifts.
Focus on getting stronger on these lifts and don't add any other exercises. Do this on a high protein diet, and you will develop some incredible muscle mass in no time.