This weekend its the new Theros Prerelease, so just incase you have missed them or are a little bit unsure here are the new mechanics ...
A creature with bestow kinda works like an Equipment, in that if you cast one as an Aura it sticks around on the battlefield even if the creature it was attached to goes away. You can also just cast it as a creature and skip the whole piggybacking as an Aura thing. So! You cast an enchantment creature with bestow as an Aura and attach it to another creature you control, which gets some spiffy bennie from that. But then oh waily waily your attached-to creature dies and... well, then your bestowed enchantment creature stops being an Aura and becomes its own person. Or Crab. Or whatever. (And visual protip: enchantment creatures in Theros have a sparkly starfield border most prominent above their type lines—and some of these have bestow!)
Whenever you cast a (presumably beneficial but not always!) spell that targets your heroic creature, your heroic creature gets better in some way. "Heroic" itself is an ability word (and as the FAQ points out, "an ability word has no meaning") so think of it there as a heads-up: "Oh hai, this duder gets buff if I target it with a spell."
Sometimes, you have enough mana to cast a cool creature and you're content with that. But forget just being content—hunger for more! Well, if your cool creature has the keyword action, "monstrosity," you can pump some mana into it and get more! Monstrosity puts one or more +1/+1 counters on the creature and possibly has some other neat effect as well. You can only activate monstrosity once, but believe you me: once will almost certainly be enough.
Okay, you know how you have some permanents (artifacts, creatures, enchantments, lands, and Planeswalkers) on the battlefield? And some of those have colored mana symbols? Well, devotion cares about those mana symbols. Your devotion to a color is the number of mana symbols of that color that appears among all your permanents. So lands don't help and colorless artifacts don't help, but colored artifacts, creatures, enchantments, and Planeswalkers all do!
Scry returns! When you scry 1, you look at the top card of your library and you have the option then of putting it on the bottom of your library as well. Scry, by itself, doesn't let you draw that card... just to look at it and decide whether or not to put it on the bottom. And some cards even let you scry for more than one! (In which case you put any number of them on the bottom of your library and arrange the rest on top of your library. But you still don't draw a card.)
Legendary and Planeswalkers Rules
Just a reminder (or maybe first-time information if you didn't know), there are new rules for legendary creatures and Planeswalkers now in effect. To wit: If you control two or more Planeswalkers with the same type (let's pick "Elspeth" as our example!) or two or more legendary permanents with the same name (like, say, Tymaret, the Murder King), you choose one and put the rest in their owners' graveyards. The new rules don't care anymore about what your opponents have on the battlefield, only you. What this means is that if I have an Elspeth, Sun's Champion I am quite happy—no wait, where was I? Oh right, if I have an Elspeth, Sun's Champion and you cast one of your own (fist bump!), they both stay on the battlefield! But if I then cast an Elspeth, Knight-Errant (what is this, Commander?), one goes to my graveyard and the other sticks around.