What are some common mistakes beginners make? 

Beginning, you should never start playing this game or you will be hooked for life, its worse than cocaine and will make your head hurt. On a serious note always assume your opponent will make the best move, never hope for a mistake and go for blood....

try to get the rooks out to early.

castle too late, or not at all

move the same piece in the opening repeatedly, or let themselves be forced to (ie 1. e4 d5 2.exd5 Qxd5 3. Nc3 ...)

another mistake chess beginners make is to set the board up with a black square in the kingside corner. this is probably the worst thing you can do in all of board gamedom

I saw a couple of guys do this the other day, I told them after a couple of moves and they asked if it was really that big of a deal, I told them it wasn't unless they wanted to play chess.  They didn't fix it.

p.s. Hi George.

Most beginners can't see threats. I mean, I once played this patzer and it was like every time I made a threat he went completely blind for one move (or several depending on how much inititave I had)

I am sick of all of you talking about me!!  How did you know my middle name was Patzer?  I quit!

I've noted beginners often make useless exchanges or go for short-term goals. Aim for the King always.

Moving without a reason to move is one I have noticed in myself and others. My rating increased quite a bit when I decided that I wouldn't make a move without a plan to go with it. It may not be a good plan, but that is for another time.

Just wanted to point out to any impressionable minds out there, the Scandinavian Defence shown here (1. e4 d5 2. exd5 Qxd5 3. Nc3) is NOT a bad opening, it is sometimes even played at Grandmaster level.

Just wanted to clear that up.

Take your time, never hope for your opponent to make a mistake, don't exchange peices without a good reason (i. e. Gaining an open file, getting good knight position)

Opening and blindness of mating threats. Also missing the pins, skewers and forks. Also unknowledge of pieces' value.

I played one player in real life.

Opening: 1.e4 a6?

End: something like this:

A lot of beginners attack too early with only their queen and/ or minor pieces. Never even think of using their rooks; except to trade them off for their opponent's rooks.

I don't think going for the king a lot qualifies as a beginner mistake. Many agressive OTB players attack the king when they see an opportunity. It is the patzers that attack without meeting the proper preconditions.

A lot of very good suggestions above that I endorse 100%.

A crash course just to add a few and give other contributors aan opportunity to post.

1. Always, always, always, you hear me now, develop your pieces. I will leave to other people to give you the sequence of what piece to develop first as it is related to the opening.

2. Be sure to protect any piece that you develop, unless you can withdraw or exchange the very next move that you make. An example is: king pawn's opening, Fisher's favorite as he almost always played this with white's,1.e4 e5, 2.Nf3 Nc6, 3.Bb5. Your bishop on b5 is not protected but you do have 2 choices, withdraw to a4 or take the knight in c6, I usually opt for ther latter as I double black's pawns on the c file. A better plan I think for the game ending.

3. Put your king in safety, the first chance you get, that is when no immediate threat is present, meaning next move or two from your opponent you will not loose a pawn or a piece or be drawn into an exchange of material that will put you behind. Not always obvious if you cannot analyze at least 2 moves ahead for all the pieces on both sides. But taking risks is how you learn.

I ain't no player, but it took me awhile to realize that when you are in a bad position, although counterattacks are good, sometimes one needs to:            (1)  retreat and regroup; or (2) lose as little as possible and then retreat and regroup. 

Qe2! a5 Qxe7++

Whopsie! :P hey it's not presice...