These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
From what can be seen, the main discussion is whether or not the overabundance of Anime Theme Song beatmaps is a bad thing.
Storyboard toggles, skin toggles, and black backgrounds are argued as either necessary for competitive players, or ruining the creativity within osu!. Peppy, the creator, has claimed he will never add a full toggle on any creative elements without having some sort of condition, which of right now is having to play the map at least once with everything toggled on.
Fan Nickname: peppy is occasionally called pepsi. Also, osu! users can add their own nicknames to their "highlights" list.
In fact, osu!'s community is very diverse, and you'll find players from a majority of developed countries in North America, South America, Europe and Asia, but not too many players from Australia. Not even anyone in the top #100 players in Performance Points are from Australia!
osu! is meant to simulate Ouendan and EBA. Unfortunately, this includes whatever Scrappy Mechanics the source games have, including spinners (which beatmap creators seem to be obsessed with, especially when making harder maps) and the very, VERY combo-based scoring system.
Now maps that abuse spinners are considered unrankable (it doesn't stop some maps occasionally being ranked when they have overly long spinners). Also, now there is the pp system, that balances scores between maps with a wide variety of max combos (but also makes plays that aren't Full Combos almost always worthless).
Modifiers multiply your score. One of these modifiers doubles the speed of the song. So to get a top-ranking score, you have to use it, which not only artificially inflates the difficulty of the beatmap, but also will probably make the song sound terrible.
More than a few players would like the option to toggle hitsounds and/or backgrounds unconditionally, a feature peppy is unwilling to implement (as mentioned above).
Playing with a mouse is this for some players, particularly those who can't afford a tablet or a touchscreen, due to the worrying potential to get carpal tunnel syndrome.