I wouldn't say these are bad shots. You are evidently very good at getting close to these snakes and your first three shots show attempts at interesting compositions. With some more practice and time spent analyzing reptile shots from professionals I think you can significantly improve.
I disagree that the top shots are taken with top gear. Very good gear often makes it easier, but many of the best shots I have seen used very low end gear. That being said, you will not be disappointed with the 100/2.8 IS macro.
It is true that sharpness and optics-wise the IS and non-IS versions are almost identical. However the IS, IMHO, completely sets this lens apart. It allows you to shoot at shutter speeds otherwise unthinkable in macro.
In terms of your last shot, this is just my opinion but I offer the following critique.
The #1 issue I see with many beginner's photographs is they attempt to put too much in the shot. This is an issue with this shot. The green tuft (moss?) is distracting. I am also unsure whether this is a portrait of the snake's head or just the eye. The nostril seems to indicate this is a shot of the head, but then it is missing another nostril. Keep things simple. Decide what you want your subject to be and remove everything else.
Your DOF also confuses slightly. I would like to see a touch more DOF - such as F8.
I believe you are following most of the proper technique already - handheld, no flash, and no AF.