Is it a good or bad sign that meditation is sometimes a frightening and nervous activity? It can happen that one gets scared or nervous by just sitting there, one can't for example sit still. But is this a good or bad sign?
Meditation is an activity of 'mental solitude', which will bring up fear &amp; nervousness. Fear is an instinctual emotion that probably every practitioner that "crosses over" must pass through when the instinctual 'self' or 'ego' is diminished. 

For people with a history or disposition of mental instability, they probably need to be careful here and should ideally meditate with a group.