I don`t know so much about Tai Chi, but I think it`s a dicipline that takes time to learn, which of course still can be just as fun as everything else.
Before I trained Karate for 2 years, and it`s very good for the mental health that too. The reason is because on this, as most of the martial arts, you exercise your inner organs, especially Solar plexus, this is something that I just could feel.
The main difference between Tai Chi and all the other Martial Arts, is the different movements compared to the others. It`s very soft movements, and according to the taoists and the yogis, these three exercises is one of the very few trainings where you don`t harm you inner organs. According to them you will actually also be harming these in methods like Karate, soccer, and whatever other trainings.
I guess the negative consequenses for the inner organs of all the different exercises can be discussed.