However, here's the reason that he is a mythic rare: as long as he's on the board, you can never take damage that will bring you below 7 life. Unfortunately, he doesn't prevent effects that make you use life. So, he's no out for a Birthing Pod deck and he won't save you from the Geralf's Messengers of the world. So, Elderscale Wurm is a marginal Standard play at best, at least at the moment.
First of all, he's a 7/7 trampler, which is good for starters. When he enters the battlefield, if your life total is less than 7, Elderscale Wurm restores you back to 7 life. The other good news is that you basically become invincible to combat damage as long as he's on the board and you're at 7 life. With mono-green being very viable in the Limited format, this guy is a definite bomb, especially in the M13 format. He's also quite good in Commander, and is a great Green Sun's Zenith target. He's just not going to save you from card effects that take life from you; the card effect has to specifically deal damage, not make you "lose life" and that's the kicker. However, he's a good mythic rare, and another good green fatty for the mono-green lovers out there.
Also, he's got 7 mentions of seven (including his power and toughness) on his card. Got to give the design team props for that. This card wins in both terms of flavor and being a Limited/Commander bomb.