In the orginial case, ie a character readies an action to move if the dragon comes within thirty feet, as long as the dragon is further that thirty feet away and declares a charge action, then the character would be able to move out of the way and the dragon could not redirect to the character. This is because during a charge action, the creature/character perfroming the charge MUST move in a straight line (barring feats). So, the dragon charges, gets within thirty feet, activates the character's readied action, the character moves away from the charge line, and the dragon is screwed. The dragon can redirect his charge to another target within his charge line and movemnet and not lose his action, however, but may not change his charge line.
Along these lines, if the character readies an action the move if the dragon moves toward him, and the dragon starts to move, this triggers the character's readied action. Since the dragon has not technically moved, it may then redirect the charge to the character's new location.
If the dragon chooses to make a move action to close to the readied character, then the ready action takes place at the stated point, but the dragon may also change direction and continue to close. This would be the best tactical action for the dragon, and, given the average intellegence of dragons, the one it would most likely take. Unless, of course, you managed to really tick it off...
As far as the abuse of readied actions goes, a character gives up an action in order to prepare a readied one, so any potential abuses end up being self regulating in the end. The example of the reach fighter reading to step 5 ft and attack when he is attcked will work once, then he has to waste his next action to move away and reset the readied action to do it again. While he is preventing the enemy from closing, he is giving up any multiple attacks he may have, and is risking the enemy doing the 'fool me once' tactic and just ignoring him. This tactic is also not helpful for the rest of the party, who are left out in the cold as the fighter runs off to set his trap the second time. It also doesn't work well against multiple opponents.