Making yogurt without heating milk?
The biggest reason to heat milk to almost boiling before fermenting is that it improves the texture of the yogurt.

During fermentation the bacteria consume lactose and produce lactic acid which causes the milk proteins to denature and coagulate trapping most of the fat. The proteins involved are primarily the casein proteins.

When this happens, there is still quite a bit of protein left that isn't bound up in the new casein mesh. All of the albumin proteins are water soluble and will not add to the structure of the yogurt. 

These albumin proteins denature when they are heated. For this reason recipes universally call for the milk to be heated to 190 and then cooled. The albumin is denatured and is able to tangle up with the casein during fermentation and add to the yogurt structure.

Skipping this step will make a very profound difference to the structure of your yogurt. Without it your yogurt will be thinner and much more fragile. When you scoop it there will be more whey and all that albumin will wash out in it.

According to Harold McGee (On Food and Cooking pp 48)

"These treatments improve the consistency of the yogurt by denaturing the whey protein lactoglobulin, whose otherwise unreactive molecules then participate by clustering on the surfaces of the casein particles. With the helpful interference of the lactoglobulins, the casein particles can only bond to each other at a few spots, and so gather not in clusters but in a fine matrix of chains that is much better at retaining liquid in its small interstices."