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.
Tween, a Japanese-developed Twitter client, is popular in its home country and is one of the top downloads on SourceForge.JP, but outside of Japan, it's obscure at best and dismissed as user-unfriendly at worst.
The upgrade to API 1.1 and the "Display Requirements" that came with it has not made some people, especially developers, happy.
Subtweeting, or replying without mentioning the user you're replying to. While some find it amusing, others find it exceptionally annoying, especially if one isn't following all participants of the conversation.
Dude, Not Funny!: Every so often, some users like to spread a rumor on Twitter that some celebrity has died when they're still very much alive.
Or even worse, joking about city council budget cuts.
After the Newtown shooting, one troll flat out crossed the line by pretending to be the shooter on Twitter (@Blastinkids) and saying he enjoyed killing them. His account is currently suspended.
Ensemble Darkhorse: Pretty much everyone is following one or two Horoscopes (usually and obviously, their own).
Fan Dumb / Hate Dumb: Good Lord, there is a reason why so many people have grown to despise Justin Bieber and One Direction even if they were, at worst, neutral to them before. Beliebers and Directioners (among others, but they stand out the most) take it upon themselves daily to start trends about them for almost no reason, get in the way of actual world events, hijack anything tangently related, and get offended en masse by other users (or even Twitter admins) wanting them to just shut up already.
Notably, "CALA BOCA GALVAO", a complaint against an obnoxious sports announcer that somehow became a campaign for saving endangered birds.
Japan is quite fond of Twitter as well; many users are big enough fans that they take features such as real-time tweet streaming very seriously. It also helps that the Japanese language allows squeezing in the same amount of information in less characters than in most Western languages, allowing one to practically write a short essay in 140 characters.
Goddamned Bats: Spambots, who once in a while will randomly reply to one of your tweets with anywhere from offers for free electronics leading to a fradulent link to nothing but a link. They're usually suspended within a few minutes, not that it matters because new spam accounts are always pouring in. This is due to the fact that Twitter, owing to its roots as an SMS service, has no human verification measures during its registration process; you can start using your account even before verifying your email address.
Hate Dumb: There are people who refuse to get a Twitter account because of excuses like "I don't want people to know I'm on the toilet", etc. Never mind that it's up to the user to determine whether or not they want to make posts like that.
And then there are those who refuse to register because they think other people's streams are nothing but shallow self-indulgent posts, so not entirely fair.
Unrelatedly, there are (or seem to be) an endless supply of hate and troll accounts on Twitter. It seems Twitter is doing very little to prevent them from taking over, aside from a "Block And Report For Spam" button underneath every account's profile page.
Hatedom: Quite a bit compared to the other big twosocial networking sites. Facebook is "one of those things that people nag you to have so you can keep in contact", Tumblr is where people go to rave about their fandoms, but Twitter is often seen as little more than a site for celebrity egos and posts that are impossible to make meaningful due to the 140-character limit.
Hype Backlash: As with any website with social networking elements, it gets quite a bit of this.
The hashtag, popularized by Twitter, is often used even outside of Twitter.
sameExplanation Used when agreeing with something the user replies to or retweets. Sometimes it's used in reply to things that wouldn't make sense to agree with, on purpose.
SubtweetingExplanation Replying to a tweet, but not attaching the @-username of the tweet. This has not been well-received by some users, as it often creates needless clutter especially if some of the participants of the subtweet conversation is not being followed by the users seeing the subtweets.
One such instance happened during the Suzuki AFC cup finals first leg where Malaysia won 3-0 against Indonesia and Indonesians accused Malaysians (which was the home team at that time) of pointing laser beam at its team's goalkeeper, despite there are proofs of Indonesians did it at first during group matches. It managed to top the trending for a few days, outdoing Justin Bieber.
Nightmare Fuel: Occasionally, you'll find a spambot who quotes one of your past tweets. That's just creepy.
Version 1.1 of its API. Among other things, it only allows clients to perform 15 API calls per 15 minutes (as opposed to the 150 or 360 calls per hour in 1.0), and all clients under 1.1 must conform to a series of "display requirements" that were not part of 1.0. This gives developers less freedom in how to design their clients, and users, particularly heavy users on mobile devices, will experience problems adapting to 1.1. Worse, every client is bound to a limit of 100,000 authentication tokens (or 100,000 users, but some people have more than one account) before requests must be made to Twitter staff for more tokens, effectively forcing a cap on the maximum number of users each client is permitted to serve.
As of July 2013, Twitter has backpedaled on some of the Display Requirements.
Putting your account into protected mode, which only allows users already following you and users whose follow requests you approve to read your tweets, is a useful way to preserve your privacy. However, it will not allow you to interact with users not following back, unless they send you follow requests or have a non-Twitter means of contacting them.
For the longest time, protected users' tweets would not show up in searches, rendering the ever-popular hashtag function near-useless to such users. However, an August 2013 update finally allows protected users' tweets to show up in their own searches and their followers' searches.
Direct messages can only be sent to people following you. While understandable in that it prevents users' DM inboxes from being filled with spam, it also means you can't have a private conversation with someone not following you (back) unless you have an external means to do so (e.g. instant messaging). However, an udpate in October 2013 allows users to, if they wish, enable DMs from anyone following them, though this comes with the risk of receiving spam messages.
Another October 2013 update brings inline image previews to web Twitter and official Twitter smartphone apps. This wouldn't be a problem if previews were optional; while a later update to the official Twitter apps allowed toggling them on or off, users of the Twitter website (rather than third-party clients) are stuck with the previews. What makes this feature a concern is that this feature can be abused to spam pornographic images. Good luck browsing your timeline in public!