Most common mistakes in zombie movies

While not the first zombie movies ever made, the original "zombie trilogy" by George A. Romero is by far the most influential set of movies of the genre in all of movie history. They popularized zombie movies, and they set all the major "rules" of zombies. And on top of that, they are really good movies.

Perhaps the best characteristic of these movies is the extreme realism they depict. Of course I'm not talking about the very existence itself of zombies in the movies' universe, but everything else. The only "supernatural" thing depicted in the movies is that the bodies of dead people, for an unknown reason, get reanimated. The bodies are still dead, they just move and have some minimal brain functions. Everything else follows very physically plausible laws, such as:

• The bodies decompose over time due to natural processes, because there's little to no immune system fighting the micro-organisms that consume dead meat. This can be clearly seen as the trilogy progresses (with the zombies in the first movie being only slightly decomposed, and by the third one they are in a very advanced state of decomposition.)
• The zombies move and act like a human would act when their brain capacity is been greatly diminished. Only some basic instincts are left. The zombies do not gain any special superhuman powers from having been reanimated (other than their bodies keeping moving even after a massive amount of damage or decomposition.)

Another great aspect about the movies is that they do not even try to "explain" why dead bodies are reanimating. They don't need to. That's completely inconsequential, and in fact makes the movies scarier and more enticing. It also works from a storytelling point of view: Why spout unscientific technobabble when you can just as well leave it as an unexplained mystery. It works much better that way.

Many, many other movies try to cash on the genre. Some are acceptable, some are just horrendous.

The most common mistakes that such zombie ripoff movies make, that usually end up just ruining the movie and not making it better, are:

• Try to explain the origin of the zombies. Why do they not understand that a good zombie movie does not need any such explanation? That it works better if it's left as a mystery. Any technobabble excuse you can come up with will only be worse than leaving it unexplained.
• Limit the "outbreak" to a small, contained place, and have the authorities try to keep it from spreading. While this can work with some types of "zombie" movies, it's usually a bad idea. It just makes it into another "disease outbreak" movie, just a much less plausible one.
• Speed up the process to a supernatural degree. In other words, make people convert to the rotten form of zombies way too fast, much faster than what flesh decomposes naturally. The worst examples I have seen made a normal person into a completely decomposed zombie in mere seconds! No, just no. The zombies in the original trilogy decomposed slowly, and it worked. There's just no need to "speed things up".
• Give the zombies supernatural powers. This doesn't make them scarier, it makes them just ridiculous. A running zombie is still ok (as long as it runs at a normal human speed), but anything superhuman is just ridiculous. The worst I have seen had "zombies" that could crawl on walls and ceilings, and were significantly stronger than normal humans. This just doesn't work, and it's one of the stupidest ideas for a zombie movie ever.
• Make killing them completely implausible and/or supernatural. This includes either making them ridiculously hard to kill, and especially making them ridiculously easy to kill (even easier than a normal human being.) The most egregious example I have seen was a zombie movie where the zombies exploded into a cloud of ash when killed with fire. I'm not kidding here. It was exactly as ridiculous as it sounds.

In short: Don't try to explain what causes zombification, it just doesn't work. Make the zombies act like dead bodies that move and have some minimal brain functionality, no more, no less. Try to keep it as realistic and natural as possible (other than the fact that dead bodies are reanimated for some unexplained reason.) Most importantly, do not go the route of giving them superhuman/supernatural powers or characteristics. None at all. It just doesn't work, and it doesn't make them scarier. It just makes them sillier.