Cooking risotto seems to differ from regular rice in two important ways:   you stir the risotto regularly you only introduce liquid a little bit at a time   What is the effect of these two steps chemically on the rice that yields such a different product than just throwing it in boiling water and covering? 
I always thought, and one of my molecular cooking books indicates that, smaller batches kept the heat higher (less temperature shock), and the scraping released starch. The article Key success factor for tasty and creamy risotto on ideas-smart.com indicates something slightly different:     Note that the instructions normally   suggest that each batch of the stock   needs to be absorbed by rice. In   reality, something else is happening.   Most of the water actually evaporates.   The remaining “mash” gets warmer than   boiling point. This speeds up the   chemical processes, which create the   intense taste of the risotto. These   chemical processes are essentially the   same as when browning meat.      Other reason for pouring the stock in   small batches and stirring constantly   is to let loose starch from the rice   kernels. Starch makes the mixture   creamy. So the tedious process of   adding stock in small batches and   stirring constantly is the key to   having tasty and creamy risotto. Only   the reason for doing this, which most   recipes give, is often misleading.   This matches personal experience and some (other) of my cook books. 