There's nothing quite like good old fashioned beatdown. The Insanity deck swarms your opponents with all kinds of beasties, and it has a few tricks to boot. Sure, thirteen of the deck's creatures cost one or two mana, but that doesn't mean that the deck has no heavy hitters. You also get some big tramplers and lots of ways to pump up your weenies when blockers appear.
Insanity differs from other beatdown decks because it makes discarding cards seem like a good idea. Centaur Veteran, Grotesque Hybrid, Krosan Archer, Putrid Imp, and Wild Mongrel are all "symbiotes" - you can discard cards to make them better for a turn. Nantuko Cultivator can get really big if you discard lands when it comes into play, and it replaces the cards you lost, too. Narcissism turns every one of your cards into a mini-Giant Growth.
You won't miss those cards you throw away. This deck softens the blow by including lots of cards with madness (Arrogant Wurm, Basking Rootwalla, and Strength of Lunacy) and flashback (Acorn Harvest and Sylvan Might). Chances are you'll be able to play the card you discarded anyway.
Spend your first few turns playing creatures like crazy. If possible, discard Basking Rootwalla to activate Putrid Imp's or Wild Mongrel's ability, then play the Rootwalla for free. In general, try to avoid paying full price for your madness cards. Instead, wait for the opportunity to help one of your creatures by discarding a card with madness, and play it at a discount.
When blockers start to hassle your army, use Muscle Burst, Sylvan Might, and Narcissism to burst through. You should be able to destroy most of your opponent's creatures in combat, so hang on to your Waste Aways until something really annoying hits the table.
If you like the Insanity deck's speed, you could take out some of the more expensive creatures, such as Grotesque Hybrid and Centaur Veteran, and replace them with cheaper ones, like Werebear. Diligent Farmhand would be a good choice, too, because it makes your Muscle Bursts stronger. You might also play some cheaper creature removal spells, such as Chainer's Edict.