It's my feeling that this should include what would be automatically understood by natives.
I have never found this to go too far - I have always wanted more local context.
I really dig for those fansubs that explain the context. Some even go extra mile by adding text frames in the video explaining why the speaker said that and what this line actually means.
We tend to miss a lot if we only have a literal translation of what the speaker have said - like a joke hidden in plain words. I remember one anime where the hero's name if written in another way meant pervert - I would have never understood why other characters used to get horrified(in a comic way) upon hearing his name for the first time if the sub had not explained it.
If you can't explain what you need to in the translation itself, just don't. Or include a TLNote.txt if you really must.
I can't speak Japanese, am not embedded within current popular Japanese culture and did not go to a Japanese high school and therefore there are references which I will not understand. These include jokes that rely on wordplay such as puns and malapropisms, and historical notes about Japanese history, geography and society which I am mostly ignorant of.
Without those translation notes, you exclude me from understanding anything but the surface of what's going on.
Others have noted the most egregious TL notes ("Keikaku means plan" and so on) but there are some other bad examples, including "This is a reference to..." (not even everyone in Japan is going to get references either) and even a few explaining what the English words they used meant.
There are many fewer these days, thankfully.
>jokes that rely on wordplay such as puns and malapropisms
While many disagree, I like translations that attempt to localise these. You can never get literal accuracy this way, but you can make a fun, entertaining and engrossing script. You might say it's not the job of the fansubbers to write the script, but it is their job to translate the one they have, and the best translations in my view are ones that take intended audience into account.
I watch anime comedies to laugh, not to read an explanation of what the joke is and why it's funny.
