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Anaerobic
Though unifying characteristics are hard to come by for all of the termite-dwelling species of protists, given there will always be some form of exception due to their variety, one thing they almost all have in common is an anaerobic life. Almost all other protists need oxygen, but these species adapted to an anaerobic life. The termite has a digestive system with three chambers. The last one, the hindgut, is anaerobic and where the bacteria and protists live that digest the wood. In order to fully digest cellulose (which is what wood virtually is), and therefore aid everyone involved, the termite's protists require an environment without poisonous oxygen.
Cellulose Digestion - Heterotrophic Protozoans
A heterotroph is a consumer in the food chain; it has to eat other animals to obtain its carbon because it cannot make it on its own. Protozoans are a group within the protist kingdom who are all motile -- they move in various ways. The protists living in the gut of a termite are heterotropic and protozoans. They need help digesting the wood (which is where the bacteria come in), and are therefore consumers just like the termite. At some point, they decided to adapt by sharing the load. Protozoans also move in various ways, with flagella the most common method. Others may use the bacteria attached to them. By doing so, they can go to the wood particles rather than wait for one to pass by them.
Water-Loving, or at Least Fluids
If an environment has water, or water can be taken from the surrounding area, then a protist will be found living there. All creatures require water to live, though they may obtain it in different ways. A termite is no different; its body has water in it the same as a human's. Because of this, the helpful microorganisms within the gut of a termite have no trouble surviving off of the water it can share with the termite, even though they had to adapt to the slight changes required.
Symbiosis, Not Parasitic
Finally, the biggest adaptation that the protists made in order to live within a termite was their relationship with the insect. While some protists in their own family decided to become parasites, several of the parabasalians decided it was more beneficial to help its host than to use it to death. With them came the bacteria that allowed them to use cellulose, which brought another layer of mutualism to the termite's gut environment. Without each other, and the bacteria they share, termites and protists would have a much harder time surviving.