Outside of Pavel's work I have never really heard anything negative about chicken. So I did some googling. The main factors I found to support his opinion were the cholesterol content and creatine content of beef is superior. The benefits of creatine should be obvious to anyone who has been liftimg for a while. Cholesterol apparently contributes to testosterone production.
This quote comes from the article below. Gives a little more insight on the subject.
"I often have lunch with Pavel Tsatsouline in Los Angeles. I'll never forget the first time I dined with him. I ordered chicken tacos. When I ordered he shot me a menacing glare and said, "Eating chicken makes you weak." Then he went on to tell me that whole eggs are good for two reasons. First, they're excellent for helping you build muscle and strength. Second, they keep chickens from being born. (Yes, Pavel has a way with words.)"
http://www.t-nation.com/free_online_article/sports_body_training_performance/5_ways_to_boost_testosterone
Hope that helps clear it up. Or at least make it slightly less cryptic.