Well my last 2.5 session hours have gone according to plan, so I'll use this opportunity to try and list some of the differences between online and live play. These are some things that I noticed or picked up on over the last few weeks. I'll add to this list as I encounter more things.
Actions that occur during a hand
string bet: A string bet is an illegal type of bet/raise. It occurs when a player starts putting chips in the pot (without saying the word raise) and then reaches back to his/her stack for more chips. The rationale behind this rule is that a player might be observing an opponent's reaction before actually committing chips to the pot. (e.g. if the opponent looks like he/she wants the raise, then the player would not raise, and vice versa) If you say the word raise, you can go back and forth to your chips as often as you like.
moving your chips over the line: Some players have a habit of holding chips in their hand. When it is their turn to act they may reach out with their hand and tap the table with the intention of checking. In some (perhaps many) cardrooms, this will actually be called a bet (if the chips cross over a line on the table, or in some cases if the chips pass in front of the players hole cards cards). The rationale for the rule is much the same as for the string bet.
throwing out an oversized chip preflop: If you throw out a single oversized chip preflop without making any verbal announcement, it will be classified as a call only. I've been burned by this before in a NL tournament when I picked up pocket As in the SB when the blinds were 25/50. 4-5 people had limped in, and I exchanged my 25 T$ chip for a 500 T$ chip without saying anything. The dealer immediately declared it as a call, and I lost a fairly sizable pot when the BB flopped 2 raggedy pair. (he instantly knew my hand because everyone at the table understood that I intended to raise, but I had no idea what he had)
verbal actions are binding: self explanatory
A common theme I have found is that you are generally better off to verbally call out your intentions. In fact, whenever you want to make a raise, I think you should always say "raise". There have been occasions in limit hold'em when I have tried to be fancy and say things like "3-bet" or "cap it", and the result was not optimal.
I have 2 examples of this: In the first hand I was on the button preflop w KK, and it had been raised in early position, and I somehow managed to miss that another player in between had reraised. When the action reached me, I said "3 bet" and ended up basically only calling the reraise. The hand ended up very multiway when I had intended to shut more people out by reraising.
In another limit ring game hand, I was in a situation where I was trapping a trapper, and, by not saying raise, I believe I cost myself at least 2 bets. I had been playing very tight for a long period of time, and just started changing gears. I raised UTG w J9s. It was folded to a tricky opponent in the BB who reraised me w KK. I am positive my opponent put me on a big PP or AK. We took the flop headsup which came KQ3 rainbow. He checked, and I checked behind him. The turn came KQ3T rainbow. He bet and I smooth called with 2nd nuts. (probably another mistake on my part, but that is not the point of this example) The river came KQ3T8. He bet, I said "2 bet", he said "3 bet", and then I stupidly said "cap it" which he just called. We were heads up so there are no limits on the number of raises. With the top set, I believe my opponent would have continued the raising. I strongly believe that when I said "cap it", it psychologically stopped him from raising. In fact I believe it cost me 4 bets or more...
acting out of turn: Most players are considerate enough not to fold out of turn. This is desirable because it does not give away free information to the player that is currently acting. However, when playing against untricky players, you can often look at the people on your left out of the corner of your eye and see that they are getting ready to fold. Against tricky players, this doesn't mean anything because they may be pretending they will fold when they will actually raise. ("weak means strong")
chopping the blinds: in a ring game, if everyone folds to the blinds a common thing that is done is called "chopping the blinds". In that case, the hand is not played, and the each of the blind players take back their chips. This is regardless of their hands. It is generally considered a friendly thing to do. I always chop the blinds simply because I want to keep the appearance of the game to be friendly (and for the same reason, I often chat with my neighbors between or during hands) even though I just want to take every single chip from every single player at the table.
showing or mucking your hand at the showdown: One of the most common mistakes I have made since starting to play live is that I will sometimes show my losing hand on the river when it is unnecessary. This is a category of mistake that I consider very stupid. I don't believe in giving away information for free. Keep in mind though that if the hand goes to the showdown, any player that was dealt in the hand can request to see the hole cards of any player involved in the showdown. However, this is not commonly done, and may be considered rude. I have seen it done less than 20 times in the first ~1000 hands I have played.
coloring up in a limit ring game: Good players that have accumulated a large number of chips will often exchange a large portion of their small denomination chips for a few large denomination chips. This is called coloring up (this kind of thing is also always done at certain stages of tournaments). In the case of ring games, one motivation for coloring up is to partially hide how well you are doing. For example, some new inexperienced player may join the table and become very intimidated to see immense chip stacks at the table. By coloring up, you can appear to have a very moderate size chip stack, when it is actually huge. (you are not allowed to remove chips from the table while remaining in the game)
live straddle: this is a strange decision that I haven't really figured out the relative merits of yet. The idea is this: The UTG player can choose to put in a blind bet that is twice the size of the BB. By doing so, the UTG player gets to act last preflop (after the BB acts), but acts normally after the flop. I have seen players elect to make this choice maybe 3 times out of probably 1000 hands that I have played. On all of those occasions, the player was on tilt. Perhaps in some cardrooms, it may be a more regular common choice..... It definitely induces more action and bigger pots.
chop: a pseudo karate chop to the table is the univeral symbol for chop. For example, you might use this action when tipping the dealer. If you toss a $2 chip at the dealer and make the chopping motion, the dealer will understand that you are tipping $1, and toss you back a $1 chip.
When to play your first hand?: When you first sit down in a ring game, you can either choose to wait until it is your turn to take the BB (which could be right away) or wait to post a blind bet equal in size to the BB in any other position (except the SB and the button). The 2 most common positions people will post in is in a special temporary position in between the button and the SB or in the CO position. You can often identify that a person has come to gamble if they will sit down UTG and post right away (instead of waiting for 1 more hand to take the BB).
It is not 100% clear in my mind what is the best decision to make if I am faced with the choice of taking the BB or posting in between the button and SB. Very many players elect to choose to post in between instead of taking the BB. However, in games where you only have to post 1/3 of the BB in the SB position, I think it is much better to take the BB than to post in between. The reason is that it only costs you 1/3 of a BB to receive 2 more hands. It is less clear to me when the SB is 1/2 of the BB. I will sometime decide to post in between with the rationale that I will observe my opponents for 2 more hands before I start playing against them (additional information on their tendencies).