'trail' shoes come in many forms (as do trails!!) - for off road running in grassy or rocky areas good grip is required - our favourites are Inov8's and Walsh (more for the fell runner really) - in the Inov8 range there are loads of choices too - some best for muddy, grassy, scree etc with really agressive soles, others better for trails with good grip and a bit more cushioning... but these shoes really come into their own on rough trails and are the fell/mountain runners faves. Salomon's get well rated as a fell shoes also.
http://www.outdoorsmagic.com/review/...-Shoes/v/1/sp/
but really - for many well maintained trails - think the Llanberis path up to Snowdon if you've been there - road shoes are perfectly fine - we use road shoes on many of the well maintained trails and really prefer to as you get better cushioning generally.
in short - you have to apply commonsense - if you're slipping around and falling over on the trails you run - you'll have to improve your grip - another consideration is protection at the front of the foot and the sides (from kicking stones or scraping between rocks etc).
beware of many of the general 'trail' shoe on the market - many of them are crap - check out the sole to see if they have suitable grip and dont even bother with gortex or waterproof - all that will happen is that the water WILL get in - but it wont be able to get out!!!
look for good protection around the toe/front of shoe and the sides - kicking stones hurts!!!
hope this has helped