Vorel of the Thallids utilizes the oft forgotten Thallid fungus tribal type, which are typically built to the following design: Add a spore counter at the begining of your upkeep, remove three spore counters to put a 1/1 saproling into play, sacrifice a saproling to do something.
I've tried many commanders for this deck type, but Vorel is the most efficient one that I've found both because it is capable of rapidly increasing the ability to generate saprolings, and thus create a large army or have a good source for the secondary effects of the thallids, but can also syngerize with other sources of counters.
This deck uses green options to support the fungal tribe, which are mostly either thallids or creatures built like thallids that are not actually called that (elvish farmer, utopia mycon). From blue it mainly gains card draw to maintain tempo, as well as various creature duplicating spells (rite of replication, followed footsteps).
While a bit slow to start out, this deck can ramp up quickly using Vorel, particularly if it manages to get out a copy of doubling season.