Left-Hand Path religions are ones that hold the physical to be as (or more) important as the spiritual. They are generally focused on balance, of body and mind, of light and dark, of good and bad.
LHP religions are fewer and less popular than RHP religions, possibly because of the diffusion of Christianity. Some of the most practiced LHP religions are forms of Satanism or Thelema.
Because of the 'balance' aspect of it, LHP practioners tend to be more open to the darker side of things, often practicing 'black' magick. They are also less acknowledging of taboos, treating nearly every decision as that of an individual. As such, they are often seen in a bad light.
Note: I wrote this myself, so I don't have any sources.
It's also worth noting that the principle symbol of the Left-Hand Path - the inverted pentagram - best encapsulates its philosophical opposition to the right.
The inverted pentagram has the spirit point facing south. Rituals performed on an inverted pentagram are properly done in 'widdershins,' or against the rise and fall of the sun. (Conventional pentagrams are done in deosil, or following the rise and fall.)
The inverted pentagram is both a perversion and a perfect reflection of its northward-facing opposite. It represents that which is antithetical to the universe's designated order.
Left-hand pathists rally behind this symbol for this, and one other reason: whereas the conventional pentagram's north-facing spirit point represents man's communion with nature, the inverted pentagram's south-facing spirit point represents his thralldom. Whereas Wicca tends to embrace the positive qualities of the elements, left-hand pathists prefer their more dramatic and cataclysmic manifestations. To some, water is a slow-running stream; to others, it's a skyscraper-height tsunami.
This accounts for the "darker" reputation it receives.
In regards to the inverted pentagram, I will also point out that in British Traditional Wicca (ie: Gardnerian and Alexandrian Wicca) the inverted pentagram is a symbol of a 2nd Degree Initiate and has nothing to do with the LHP, nor is it considered a perversion of the upright pentagram.
Wicca is a right-hand path? I've seen many Wiccans who wear inverted Pentagrams (In fact they sell more of those than upright ones at a local Pagan store I go to) so wouldn't that count Wicca as a left-hand path?
No, Wicca would not be considered a left-hand path. Wiccans are not the only ones to wear pentagrams both inverted and upright. What those symbols mean in those different paths can be very, very different.
In the First Degree in Traditional Wicca the pentgram (or more accurately the pentacle) has an upward point symbolizing man reaching upwards toward Spirit and the Divine. At 2nd Degree the point is downwards, symbolizing the Divine/Spirit reaching downwards towards man. This represents the duty of the 2nd Degree to begin to manifest the Divine in their daily lives.
That symbolism and meaning is very different from the symbolism and meaning for an inverted pentacle in Satanism or other LHP practices.
