Useless and/or Ineffective Techniques

Two-Handed Choke
First of all, it eliminates the use of the primary weapons (the hands) while leaving the entire body open to counterattack. Second, it is mechanically inefficient as it provides zero pulling power, which makes the hold rather easy to escape.

Spearhand
The technique itself is not the problem; the problem is how it's commonly taught. Because it's performed at chest level in many katas, it's generally assumed that the target area is either the chest (namely the sternum) or the abdominal area. Common sense dictates that striking any bony area with outstretched fingers is just asking for trouble. Striking anywhere other than the throat would cause more damage to your fingers (and possibly the hand and wrist as well) than it would to your opponent.

Standing Headlock
Primarily for the same reasons listed for the two-handed choke. An additional problem is that due to the position of the opponent, many counterattacks can come without being seen beforehand.

Blocks
Check out this previous post for the long explanation. Here's the short one: blocking does nothing to prevent the attack from stopping. Also, because of the close distance in street fighting, there isn't enough time to block in the first place.

Roundhouse kick using the instep
Ask a muay thai or old-school taekwondo practitioner why this is ineffective. The instep contains some of the most sensitive (anyone ever had their foot stepped on before?) and easily breakable bones in the body. Anyone who's ever had their kick block by their opponent's knee muay thai-style knows what I'm talking about. The better alternative would be the bottom of the shin or the toes (while wearing shoes of course).

High Kicks
I think this horse has been beaten to death.