Common areas of pressure:
Bars The naturally toothless portion of the lower jaw.
Lips The corners of the mouth are incredibly sensitive. As such, this can be the gentlest or the cruelest type of pressure. Any direct rein use with a bit will put pressure on the lips. Gag bits will exaggerate this effect.
Tongue Most bits will apply some pressure to the tongue, but it should not be the only or sustained point of contact.
Side of muzzle Usually the easiest pressure to introduce to a young or headshy horse, which is why full-cheek and d-ring snaffle bits, and side-pull and bosal bit-less options, are so popular for starting horses.
Bridge of nose Pressure varies considerably based on placement.
Curb Under the chin. This pressure can be very subtle, or vice like. As an indirect pressure, it is not immediately "horse logical," and should be introduced systematically to a horse that is already well accustomed to a rider and more direct pressures.
Poll Top of the head. This can be a very nuanced addition to horse/rider communication, but it can also be very frightening to the horse. Many "humane" rigs, including the Dr. Cook's Bit-less Bridle, have an incredible amount of poll pressure and can cause horses to panic.
Palette Either accidentally, from a single-jointed mouth piece with both reins used strongly, or purposefully from a ported mouthpiece. A ported mouthpiece is not necessarily a more severe bit, but there are many very severe ported bits.
I am often asked about bits I love or hate, so for articles to follow I will tag them:
Green: Suitable for everyday use, green horses, and green riders. Generally smooth mouth snaffles and direct pull bit-less options.
Yellow: Suitable for everyday use and green riders, but requires some training for the horse. Most simple curbs, including bit-less options.
Orange: Not suitable for everyday use, probably not something a beginner should use, requires training for the horse. This is a pretty large category, ranging from double bridles to gag hacks, to spade bits.
Red: Why, oh why, does this exist? I generally reserve this category for bits that will cause immediate pain and/or damage even in sensitive hands. If it can be used without causing pain or panic, even if it is not something I would use, I'll stick it in orange.