Live poker is played with real cards and chips. You have to look at your cards and remember them, they aren't constantly displayed in front of you on a screen. Might seem trivial, until you see a flop come with two spades at a table filled with new live players, and everybody immediately checks their cards to see if they have spades, after which some have looks of disappointment and some are betting with a new gleem in their eyes. This ain't gonna happen online.
You always have the whole betting with chips thing. Online, you type a number or slide a bar, click, and you just bet $10.75. Live you have string bets, single chip rule, misstacking chips, fake pumps, etc. Online you also have numbers all over the place. Each player's balance is displayed neatly in front of them, while live everybody has random stacks of chips (and you will have to notice that guy has two black chips sitting on top of that stack of red). The bets are displayed as little number, not a string of chips spewed across the table that you have to count, and the total in the pot is another neat little number, not a massive scattered pile of chips that you should have paid attention to how much had been bet previously. Nobody is going to tell you how much is in the pot in a live game.
Other things that happen in live games that are impossible online -- players act or try to act out of turn, cards inadvertently get exposed, people fold out of turn, the turn card gets dealt prematurely. Never gonna happen online.:dontknow: