Mulligan

Always keep: Acornbearer, Dreamway Guardians

Keep if you have at least one of the above: EVIL Cable Rat, Landscaping, Microtech Controller, Power Of The Wild

Keep against aggro: Wrath, Swipe 

Keep if going second: Wispering Woods

Replacements

Crystalsong Portal would be the first to go if you wanted to replace anything.

If you face lots of aggro, add Zilliax and maybe Wardruid Loti.

If you face lots of control, add Mark Of The Loa and maybe Tending Tauren. 

If you face lots of weapons, add Acidic Swamp Ooze.

Force Of Nature would also be good in the deck.

You could run the Treant package (Drendrologist and Mulchmuncher).

Some people use Eccentric Scribe but I personally find it to be too slow.

Some people use Leeroy Jenkins but I personally find Savage Roar to be a much more effective finisher.