Here is a brief summary of differences between the FIDE Laws of Chess and the USCF Official Rules of Chess [as of 23 May 2012]. This is not meant to be a comprehensive list.

There are two sections: "important" differences and "obscure" differences. The differences labeled "important" are more likely to occur in tournament play.

Note: All rules are presented from the FIDE point of view. (I assume the reader already knows the corresponding USCF rule.) So, for example, the first rule in the "Important differences" section should be read as: "According to the FIDE Laws of Chess, the arbiter can call fallen flags without a claim by the player or the opponent."

Important differences:

1. The arbiter can call fallen flags without a claim by the player or the opponent.
2. The arbiter will correct all observed rules violations (such as illegal moves and "touch move" violations) even if the opponent does not make a claim. (However, the opponent may make a claim if the arbiter does not observe the violation.)
3. The Laws of Chess require that a player whose cell phone rings shall lose the game on the first offense. Players are forbidden to have cell phones that are not turned completely off without permission of the arbiter.
4. You must make your move on the board first and only then record the move unless you are claiming a draw by triple occurrence of position or by the 50 move rule (or sealing a move).
5. The penalty for the first two illegal moves is to add two minutes to the opponent's time (whether or not the time control is a sudden death time control). A third illegal move results in loss of the game.
6. There is no limit how far back in the game an illegal move may be corrected, unlike the USCF limit of 10 moves or 2 moves in sudden death time pressure.
7. You must continue recording moves if you have at least five minutes on the clock, even if the opponent has less than five minutes. If the time control has an increment of at least 30 seconds per move, both USCF and FIDE rules require both players to record moves at all times.
8. Unless specified otherwise, a player who is late at all for the start of the round forfeits (the "zero tolerance" rule). The rules for a tournament may specify a different "default time," such as the USCF rules allowing a player to be up to one hour late. In this case, if both players are late, all the elapsed time comes off White's clock (instead of splitting the elapsed time equally).
9. It is not necessary to have a complete scoresheet to win on time in a non-sudden death time control. Calling your own flag to prevent the opponent from filling in moves in an incomplete score sheet won't help.
10. When castling, the player must touch the king first (or the king and rook at the same time). If the player touches the rook first, castling with that rook is not allowed, and the touch move rule is applied to the rook.

Obscure differences:

1. The penalty for an incorrect draw claim is to add three minutes to the opponent's time, not two.
2. If claiming a draw by triple occurrence of position or the 50 move rule, you must write your move on the score sheet without making the move on the board. If you make the move on the board, you are no longer considered to be "on the move" (even if your clock is still running), and a draw claim will be rejected.
3. Score sheets must be brought up to date at the end of a non-sudden death time control. If one player must complete his score sheet, he does so while his clock is running before he makes a move in the new time control. If both players must complete score sheets, the arbiter may assist, and both clocks are stopped during the reconstruction.
4. When promoting a pawn, the choice of promotion piece is final as soon as the piece touches the promotion square (even if the player has not yet released the promotion piece).
5. It is illegal to use an inverted rook to mean "queen" when promoting a pawn. If a promotion piece is not readily available, the player must stop the clocks and ask the arbiter for assistance. (Technically, I think an arbiter would be within his rights to rule that the player had promoted to a rook. I don't see anything in the Laws of Chess that require the base of the piece to touch the chess board!)
6. It is considered an illegal move to leave a pawn on the last rank and then to press the clock without replacing the pawn with the intended promotion piece. The opponent will be awarded an additional two minutes in this case.
7. A player with less than two minutes remaining on the clock may claim a draw under Article 10.2 (the vague equivalent of USCF rule 14H, "insufficient losing chances"), even if there is a delay or increment. The player may claim a draw based on the opponent not making any attempt to win the game by normal means. The arbiter's ruling on such a claim is final and is not subject to appeal.
8. It is possible to lose on time in situations that are a draw under USCF rules. For instance, GM Nakamura lost on time with a king and rook vs. king and knight. Under USCF rule 14E2, the game would have been a draw (opponent has only a king and knight or a king and bishop). Under the FIDE laws of chess, the game is drawn when one player runs out of time only if there is no legal sequence of moves by which the opponent could checkmate the player. Since there is a helpmate that allows a king and one knight to checkmate a player with a king and rook, GM Nakamura lost.
9. When correcting illegal moves, the arbiter may use his best judgment to determine the time on each clock. (USCF rules allow no adjustment of time when correcting illegal moves.)
10. If the players start the game with the colors reversed, the game continues unless the arbiter rules otherwise. If the game started with the pieces incorrectly set up, the game is canceled and a new game played in its place. (No swapping back a king and queen that are swapped and continuing [which is also incorrect under USCF rules]!)