*Many self-defense techniques are taught and practiced with other martial artists.
The problem with this is that on the streets, an attacker will likely not be a martial artist. They will not step forward and punch with a lunging punch and just sit there, nor will they throw a perfectly chambered front kick, and most importantly, they are 1,000 times more likely to resist against your techniques than, say your classmate.
*Many self-defense techniques try to predict what the assailant will do.
How many times have we heard this one, or something similar: "If the attacker does _______ (insert random attack here), then I would do ______(insert response technique here), and then he will do ______, and then I can do______." Common sense should tell us that there is no way to predict what an attacker will do, let alone how they'll respond.
*However, predictable responses should be considered.
There are some responses that can be realistically predicted. Examples include bending over after being hit in the groin, and closing the eyes and/or covering the face when something's coming toward the face.
*Many techniques are taught at unrealistic distances.
If most fights begin within grappling range, then why are so many techniques focused on attacks coming from so far away that even the most inept fighter can react?
*Many techniques require too many complex movements and/or too much thought.
Any technique that requires thought under duress is sure to fail when applied in a real situation. When in doubt, simplicity is the best answer.