On July 13, 2009, the rules of Magic will change slightly. These are mostly small changes that won’t be noticed, but some are importand. I’ll summarize each one, but if you want to read everything, there are detailed descriptions here.
1. Simultaneous Mulligans
Starting July 13, mulligans will no longer be taken be one person first, then the other person. Instead, everyone will take announce their mulligans at the same time and take them simultaneously. If you didn’t know that you were supposed to take the mulligans in a certain order, than this won’t affect you at all.
2. Terminology changes
This is a change that won’t affect gameplay at all. In play now is the battlefield, spells that are played are now cast, abilities are activated, cards that are removed from the game are exiled, and abilities that happen at end of turn now happen at the beginning of the end step. (While things that last until end of turn end after those things)
3. Mana Pools and Mana Burn
Mana burn is gone. Also, mana polls empty after every step and phase, instead of just emptying after every phase.
EDIT: I just realized that this invalidates the poll… I’ll close it now.
4. Token Ownership
Instead of the owner being the person who controlled the effect that put it into play, the owner is now the player whose control it came into play under.
5. Combat Damage No Longer Uses the Stack
This is simple. This eliminates a ton of tricks, and Mog Fanatic is now much less useful. Also, the way multiple blockers are handled has changed. Instead of the attacker assigning damage as s/he chooses, s/he chooses the order in which the creatures receive the damage, and damage is not done to the second creature in line at all until enough damage has been done to he first creature to kill it.
6&7. Lifelink and Deathtouch no longer use the stack
Both of the effects happen as a state-based effect as soon as the damage has been done. This means that if you are at 1, and block a 3/3 with a 2/2 with lifelink, and you let a 1/1 go through, you will not die. Under the old rules, lifelink would go on the stack, but then you would die before it could do anything. Also, 2 instances of lifelink now create only one effect. (For example, under the old rules, if you played Titanic Ultimatum on a rhox war monk, you would gain 14 life from the attack, but now you will only gain 7.)
Now for the preview cards. They are also in the article:

This is the new nonbasic duel land. It looks like Wizards is printing more and more nonbasic land hate, and they actually managed to print nonbasic hate on a nonbasic land with this card. This is probably a cycle of 5 lands with the same effect, only different colors, so this card is actually 5 preview cards in one.

This is a 2/1 with lifelink. Not a big deal.

This card is also not a very big deal, because it’s a 2/2 which will destroy one creature, and one artifact, enchantment, or land.

Any deck with enough skeletons, vampires, and zombies to make this useful will also have enough removal to destroy it as soon as it comes into play.