There are many things that can make a pasta great. It's not always the same things either. These are the elements I look for, and any of them may cause your pasta to be deleted, depending on just how poor the pasta was in any/all of these areas.

1. Description

A pasta may be great just because whatever is causing the turmoil is described really well. That means that maybe just a paragraph, or just a couple sentences make the pasta. While it may only be a few sentences long, it seemed to grab the reader and hold them frozen. I would have to say that most great pastas fall into this area. They are great because of the description.
If you want to know just what I'm talking about, read these pastas, I've hand-selected them to demonstrate excellent description:

2. Conclusion

Sometimes I'm reading a pasta that I really don't even enjoy until the conclusion. Then, I understand why there were certain elements earlier. Then the end ties it all together and makes me think, "Wow, that paragraph was important after all." Usually, that's very difficult.
Here's some pastas with great conclusions:

3. Composition

Composition is the originality of the concepts and events, and whether they are realistic or seem out of place in the storyline. If there are interesting things that happen, it makes for a better story.
Here's a few pastas I've chosen to highlight great composition:

4. Transition

Some pastas I have read have a pace, it's like the writer knows just how to grab your attention and when to deliver the turmoil. This brings me back to how difficult it is to write a good conclusion. A lot of pastas I have seen have abrupt endings, most likely because the writer reached a certain point, and got tired of writing, or decided to start a conclusion. When, in fact, an excellent conclusion is made by planning the end well before it actually starts. These elements are called transition. Also included in transition, is the shape of the sentence and paragraphs. This all depends on how it sounds as the pasta is read. ChristianWallis explains this well in a writing advice article: A Look at Style and Structure.
These pastas all have awesome transition:

5. Scare Factor

Then, there is overall creepiness, a measure of just how unsettling the writing was. What makes a pasta even better, is if they can relate that feeling to the reader, to where the reader feels like it may happen to them or someone they know. Also, sometimes there is just an unnerving atmosphere that makes a story great. In this way, it sometimes blends with description and composition.
Following is some examples of spectacular scariness:
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