Effective July 13th, 2013 (M2014 Release)
The following is an overview of the new rules changes that are set to take place on July 13th, 2013 in conjuction with the release of M2014. Follow the link here for a complete explanation of the new rule changes.
The "Legend" Rule
The "legend rule," rule 704.5k is changing. Under the current rules, any time two or more legendary permanents with the same name were on the battlefield, they would all be put into their owners' graveyards as a state-based action. Under the new rules, any time two or more legendary permanents with the same name are controlled by a player, that player chooses one of them and the rest are put into their owners' graveyards as a state-based action.
The "legend rule" now looks at each player individually. It doesn't matter what any other player controls. If you control a legendary permanent, having another one inter the battlefield (by playing a second one or creating a copy of the first one) will leave you with one on the battlefield. It may be the old one. It may be the new one. That's up to you.
Also, creating a copy of a legendary permanent controlled by another player will simply give you a copy. The one controlled by the other player won't explode, won't leave the battlefield, and really won't be affected at all.
The "Planeswalker Uniqueness" Rule
The "Planeswalker uniqueness rule" is getting a very similar update to what the "legend rule" got. The new rule 704.5j will state that if a player controls two or more Planeswalkers that share a Planeswalker type, that player chooses one and the rest are put into their owner's graveyards as a state-based action. Again, you no longer have to worry about what the other players control.
Sideboards in Constructed Tournaments
We're also making an adjustment to sideboard composition in Constructed tournaments. Previously, your main deck was sixty or more cards and your sideboard was either fifteen cards or zero cards, indicating you weren't using a sideboard. With the new rule, your main deck is still sixty or more cards but your sideboard is now up to fifteen cards. Additionally, you are not required to swap cards between your main deck and sideboard on a one-for-one basis For Games 2 and 3 (and so on), as long as your main deck is sixty or more cards and your sideboard is no more than fifteen cards, you're goo. This change makes sidboarding in Constructed and in Limited closer.
The real benefit of this change is the following scenario, which is all-to-common at more competitive events: after Game 1, you bring in some cards from your sideboard, shuffle up, present your deck, and you discover you've presented a sixty-one-card deck. There are fourteen cards in your sideboard and you've just received a game loss.
Under the new rules, what you did in that scenario is legal and you can go about playing Magic.