What, exactly, is in worm castings?

This is one answer. It's not referenced, so I can't be sure it's totally accurate but it sounds right from what reading I've done so far.
From this site.
The bacterial population of a worm cast is much higher than the bacterial population of either worm’s gut or ingested soils.

One of the important components of worm cast is the humus, a complicated material formed during the breakdown of organic matter. The humic acid in humus provides many binding site for plant nutrients, such as iron, calcium, potassium, phosphorus and sulfur. These slow release nutrients are stored in the humic acid molecule in a form readily available to plants, and are released when they are needed by the plants.

Worm cast also contain worm mucus which will keeps the nutrients from washing away. These mucus acts as glue to agglomerate soil particles. It holds moisture better than plain soil. Worm cast can hold 2-3 times their weight in water. It will improve the water holding capacity of the soil. In other words, your soils will remain dampen for a longer period of time.

Worm cast are rich in microbial life which helps break down nutrient already present in the soil into plant available forms. It also enriching soil in micro-organisms, adding plant hormones such as auxins and gibberellic and adding enzymes such as phosphatase and cellulase.

Worm cast improve soil structure, porosity, aeration and water retention capabilities. The product can insulate plant roots from extreme temperatures, reduce erosion and control weeds. It is odorless and consists of 100% recycled materials.

Worm casting contains 5 times the available nitrogen, 7 times the available potassium and 1.5 times more calcium than found in 15cm of good top soil. It will contains all the necessary nutrients when the wastes fed is well balanced (e.g., fruit peels, vegetable scraps, coffee grounds, garden wastes, grains, breads, small amount of egg shells and bones, shredded newspapers and cardboards.)