I ride our trails with a helmet, glasses, gloves and knee/shin gaurds.
The trails are steep and rocky and I have spikey platform pedals.
Glasses are pretty much an essential as I have had mud off the tyres thrown into them and tree branches slap across my face. A helmet is also essential, shin gaurds become essential if your pedals are spiked for grip and you don't like scars.
If you feel more comfortable/safer wearing full armour go for it, I have been tempted to go that route myself after reading about a few of the injuries some others have recieved.