It is very infrequent that a film has a huge impact on the genre as a whole. Certainly nearly every horror film has borrowed something or has been influenced by previous films. But for a horror film to influence and start a whole new style of movie, even outside the genre, is very rare. The Blair Witch Project did exactly that in 1999.
The promotion of the film was nothing short of genius. How they kept a secret so well is an amazing feat in itself. Most people likely did not believe that the film was "real" but it is amazing that none of the secrets or personal information about the film or actors was released ahead of time. As you probably recall, this is the first major "found footage" film and was marketed as a true story. The film started a craze before it even hit theaters.
As far as the film itself, it is quite realistic which tells you something about the quality of acting. The actors/actresses do look quite natural throughout the film. There are a few lines that stand out as feeling scripted, but the movie overall maintains excellent quality acting.