An effective art (martial or otherwise) is one which simultaneously allows for your manifestation and your liberation in more than one realm.
To be manifest in only the realm of the art itself is to be esoteric and elite, and is almost academic at best. At the other end of the spectrum, Tiger Woods example in golf is one which is so transcendent across realms that people of all ages from all walks of life can percieve his greatness in the mastery of that art.
One need not be as shining an example as Tiger to be effective in ones chosen art or arts.
The key metric seems to be: are you benefiting from the practice and process of the art, and in such a way which is healthy to you, those around you, and your society.
When we consider martial arts, I am of the opinion (based on what was taught to me) that the "best" martial artist is the one who never has to use his or her skills. In other words, no matter how deadly you think you can be, you do not employ such techniques. That amounts to a great responsibility, and is one definition of being an adult-- even a Master.
I would say this is a more taoist/buddhist non-violence stance, for surely when the Japanese practiced with their tachi on prisoners to test the abilities of their technique or the sharpness of their blade it contravened this concept... but then again the Japanese are only recently known for non-violence.
One modern problem in the practice of ancient martial arts is that the gun, the atom bomb, etc. have made irrelevant (or much less relevant) the role of the skilled soldier. So when measuring this pitch we see that the playing field is not level at all. Hence, nowadays, good MA practice is really eccentric or esoteric physical exercise.
However, this all leads me to my point which is to answer "what makes a martial art effective?"
Once the student has learned the realms of practice and "Hard Work" over time, or Kung Fu, and knowing basics, or even up to great detail, of the Buddhist orgins of Kung-fu, as such, we enter into a state of Mindfulness.
Musashi's Go Rin no Sho, Book of Five Rings, talks in great detail about how the perfected warrior is one who masters many skills, trades, and arts in order to gain advantage in combat. Yet his were times filled with conflict and battle which brought to bear the need for deuls to the death.
I do not pine for those days. I am glad we have modern advantages.
Yet we can still learn from those masters of old.
The bottom line: In all things find your art, and from all things let your art find you. That is the Way of the Modern Martial Artist.