Slow motion is fine as a tool. Like any other effect, I just get annoyed when it is overdone. When you do something too much, it becomes common, and expected and not special. Also - there should be a narrative reason: why are you slowing this down? Is there something you need the audience to see? Images, just like words- should have a narrative reason for being there.
You mentioned 300 (ugh.) - does the slow mo and speed ramping serve A) a narrative purpose or B) Zach Snyder thought "f**king hell, that looks badass, man!"? Again - as tool, nothing inherently wrong with it. Problem is- he ends up using it every five seconds! I think if 300 were brought back to regular speed, the film might clock in at 70 Minutes! The way it is with all the speed ramping and awful green screen- I just can't watch it because it just looks like a pretentious French perfume commercial crossed with a Car commercial by using
slightly better Weather Channel Chroma Key! ("Expect rain, clouds and look out for a front of Spartans pushing up from the south")
View attachment 394159
The tools of SFX are fine, but people are abusing, and overusing them. Remember when destroying a city was a HUGE thing?
Now, it's every other blockbuster, not special (actually, kind of a yawn). And does anyone really need to see one more killer Tsunami wave knock down a city? Been there. Yawn. The special ceases being special and becomes maudlin through sheer repetition.
Too much of anything special makes it less and less "special". Ice cream is awesome- but you can't make a meal out of it.