Below is a short list of stuff to be avoided (in general terms, not absolutely) when writing fiction.
Suddenly/unexpectedly. Ironically, these two words rob their meaning from any sentence they start. If you tell the reader a sudden thing is about to occur, the thing [when it does occur] is no longer sudden.
Adverbs. Adverbs are words like ‘swiftly’ or ‘desperately’. They don’t have to be avoided altogether (particularly in speech) but overusing them is a sign you’re getting the nouns/verbs wrong. If you use ‘hobbled’ you don’t need to point out the walking action is taking place ‘slowly’. The verb ‘yearned’ indicates the character ‘desperately’ wants something. [However, adverbs can be used for comic effect**, but still shouldn’t be overdone].
Names that are impossible. In fantasy/sci-fi there can be a temptation to have names which are highly original, which can make them bloody hard to read, let alone pronounce. It’s just annoying for the reader (although you could give such characters a sensible nickname).
Unnatural dialogue. “I had a shock encounter today. I was walking in town when I bumped into Nicola Sturgeon, who is the First Minister and also leader of the Scottish National Party.” Unlike Ed Miliband, your characters need to be able to speak human, even when conveying information to the reader.
The Never-Ending Sentence. Generally, short sentences are better. They’re simpler, easier to understand, and put across a greater sense of pace. Reading a book should be as easy and relaxing as listening to music. [Except for when you’re having innocent people brutally murdered, obviously**].
Don’t be precious. Your work isn’t the One Ring, and you’re not Gollum. Everything anyone writes is riddled with imperfections and room for improvement. Of course, you don’t have to act on everything a beta reader says (especially when two beta readers have diametrically opposing opinions on the same sentence. Or chapter) but you should consider every comment they make. The odds on the first draft being the finished product are bloody long.