Splashing the pot. In most games, splashing the pot annoys players. If you are playing with people who are very laid back (or drunk) then it might be entertaining.
Acting out of turn. At an informal game, acting out of turn is annoying but probably won't piss anyone off. But it is definitely a major etiquette gaffe at a formal game. Everyone acts out of turn at some point though, and if you do so because of an honest mistake, then the other players will be tolerant.
Folding out of turn. Folding out of turn is less annoying than acting out of turn since most people fold most hands, but at a formal game, it will still be perceived as rude. At an informal game, folding out of turn is semi-common and might not be considered rude - if you have something important to do, like answer the phone, then people will understand. One thing some players do instead of folding out of turn, is to whisper to the person sitting to their left to fold their hand when the action comes to them. Technically, this is still giving that particular player an information advantage, but if that person ends up folding then it pretty much doesn't matter.
Rabbit hunting if it is not allowed. This is not allowed at a form game. It is not considered rude as much as it is annoying. At informal games it is a very common occurrence.
Turning over another players mucked cards without his permission. This is considered very rude at a formal game. At an informal game, it may still be considered rude. You shouldn't turn over another player's cards unless you know that he won't mind.
Reading the board out loud. In a serious game, some players won't like this because you are essentially helping out players who may have misread the board. It may seem unlikely that a player wouldn't read the board correctly, but there may be particular boards that players don't interpret correctly. For example, the board in Hold 'Em is 2-2-3-3-A, then any kickers won't matter. Some intermediate-level players may miss this.
Not paying attention to the hand. Sometimes a player will be talking to other players, and when the action gets to them, they will ask what the last bet was. In a formal game this will be considered slightly rude, but in a social game, where people are always talking to each other, it will definitely not be considered rude.
Not being at your table when the action is to you. If you are away from the table and it takes you a couple of seconds to get back, then it's not a big deal. If you are away for a long time, or being generally neglectful of the game, then it will be considered rude.
Talking on a cell phone. At a formal game, it will be considered rude, but not at an informal one. Also, if you are playing poker with a much older crowd then it'll be more likely to be considered rude.
Waiting too long to act. If you are a pure beginner, then the other players will generally be tolerant of you taking a while to decide what to do. New poker players have a habit of believing that every starting hand is a major decision. If you are not a beginner, then people will be less tolerant. If you need a lot of time to decide, such as a big decision in a no-limit game, then you can call time to at least let people know you will be thinking for a minute.
Taking too long to decide on a hand because you are busy stacking your chips from a big pot you just won. This is something you see everyone do once in a while. Whenever you do this, one of the other players will usually say, "Stack your chips later". This is a small faux pas.
Slowboating. This is just a rude behavior no matter when you do it.
Talking about hands. This is a good example of the differences between formal and casual games. At a formal game, talking about hands will be considered either rude or, at least, annoying. At a casual game, it definitely won't be considered rude, and is actually very common.
Talking about hands with only certain players. In some games, talking about hands is allowed, but this doesn't mean that you should talk about your hand with only certain players. In this situation, you should talk about it with everyone. For instance, I have played in games where a player ("player A") folded his hand, and another player ("player B") will whisper to the folder ("player A"), "What did you have?" After Player A answers back in a low-tone, another player ("Player C") overhears Player A & B talking about what hands they held, so Player C will ask Player A to openly tell the table as a whole what hand he had. It should be recognized that talking about your hands with only certain players could, technically speaking, be considered a form of collusion (admittedly "accidental" collusion). Most players won't care, but some anal ones might.
Criticizing a player's play. At formal games, you generally shouldn't criticize another player's play. One of the general rules of formal games is to mind your own business. At informal games, it will depend on the personality of the player. Some players don't mind hearing about the wrong move they made, while some bad players don't want to hear about how bad they are - they enjoy chasing inside straight draws. This kind of conflict happens often when you have intelligent players (who know a lot about odds, poker theory, and math) that are playing in a game with bad poker players who have no knowledge about basic poker math. If you quote poker theory to a not-so-smart player, you may come off as an elitist.
Betting blind. Betting blind (throwing out a flop bet before the flop cards are dealt) is a move that is frowned upon by some players. Although it isn't considered breaking the rules, it is considered a move based purely for the purpose of psychologically intimidating the other players.
Bending the cards too much. Try not to bend the cards too much when you peak at them because they will wear out quicker. If you do this, the host of the game may say something to you.
Sitting next to players you know. Some players don't like it when 2 players who know each other will sit next to each other at the table (or sit at the same table in a multi-table tournament) because it increases their ability to engage in collusion by talking about hands, or passively colluding by "team playing" their hands if they are at the same table.