Other than synopsis where if well put together it can determine whether you are going to watch the drama or not. What is considered a spoiler? I got into this debate about how this girl displayed in vivid detail on this currently airing drama and she doesn't see it in a category called SPOILER ALERT. 
Therefore i was told to not read the comment section. I am not that freaked out by spoilers. Shit i sometimes watch youtube Mv's when i am looking for a drama to watch.
What is a spoiler?
A spoiler is anything that gives away major plot points or important scenes. Whoever told you not to read the comment section was both right and wrong. Right because there are always people who post spoilers there but wrong because they are not supposed to and, in theory, everyone should be able to read the comment section without being spoiled. That is why the spoiler tag is there. If someone doesn't tag a spoiler a spoiler, they are in the wrong. 
It depends on the person what is considered a spoiler. I know a few people who love to know how a drama ends before they even watch it. I find that strange...Sometimes a simple picture of a drama can be a spoiler. What if that character was supposed to die? But we see a pic of them in episode 14? Then that will just get your mind working wondering what could have happened etc. You can't know what a spoiler is to someone, but it's best to avoid the really big ones (like plot twists, secrets, etc)

I once saw a simple name in caps...ruined a huge part of a drama for me. (That's me though lol. My brain is too good at putting 2 and 2 together....) I think it's pretty selfish of some people to deprive others of the joy of discovering for themselves what happens in a drama. Did anyone spoil it for you while you were watching? Then why must you spoil it for others? ("You" isn't aimed at anyone particular ^^')
It is like someone smoke a cigarette next to you, while you are eating something delicious.
I actually hate spoiles! In general, I love asian dramas because there is always a unpredictable point: A twist, a character or something else outstanding. If someone tells me this facts before I already watched it, it ruins the whole drama experience for me.
 
An example:"The end was shocking and made me cry" my thoughts: thank you! at least YOU was surprised and emotionally involved... because I know there will be no hope, I will not feel it.
   
There will always be people who cannot understand that other people are different. If they don't care for spoilers, they don't understand why others do.They don't even get this thought. They just want to share their feelings even if they annoy others with it. So they are not evil or mean... just kind of ignorant and not empathic... even so... it is unfair :(
From yourdictionary.com:
The definition of a spoiler alert is a message before a presentation, movie, book, article or online post letting the reader know that important parts of a storyline will be revealed.