Legs too rigid. A straight or even slightly bent rear leg is too tense and hampers quick movement. To move, the leg must relax and tense again.
Legs too wide apart
"The way of moving the feet is none other than that of ordinary walking." -Miyamoto Musashi.
"Anyone who separates his feet too far will never become a good practitioner." -K. Sawai.
To move, the feet must be brought closer together or the body will be unstable. In classical karate, the feet were kept much closer together. Wide separation between the feet was introduced as a means of strengthening the lower part of the body.
Are these criticisms valid? Maybe; however, the front stance is not used in sparring; it is only used in patterns, where precision, raw power, and artistic expression are paramount. For sparring, we use a fighting stance that facilitates relaxed, quick movement and quick snapping techniques. Taekwondo practitioners are not stupid; they use the right tool for the job at hand.