Before I start, I better explain what I call which.. since I'll just translate from my swedish mind, where snaffles never have shanks etc :P
Snaffle: Rein attached to the rings and directly to the mouthpiece. No shanks. The mouthpiece can be straight or jointed.
Curb; has a chain and shanks. Mouthpiece can be different as well as length and form of shanks. (the escauela to the left has no chain in the photo, but you can see that it's supposed to) The cheekpiece of the bridle is above the mouthpiece, the rein is below. (I know the pelham allows you to have a rein in the level with the mouthpiece, but I still say it gives a very slight curb effect.)
Gag: has no chain or strap and the mouthpiece can run on part of the shanks. I concider most western curb bits without a chain more of a gag than a curb, even if the mouthpiece's firmly attached to the shanks.
That's just to make sure you know what I mean, and these are the main types of bits in my world, all bits can be put in one of these three.
The snafle work directly from rein to mouth, the same preassure you feel in your hand, is in th horses mouth. That makes them gentle, in a way. The effect is that they give the horse an impulse to raise it's head (yes, you can make the horse put the head down and forward on them, but that's more with your seat han the bit, and it's not the most obvious for the horse.)
The curb works indirectly, you feel the rein much lighter than the horse feels the bit, and you have to remember that. Italso gives the horse implses to stuck his nose in, his forehead out and his neck down. You use the curb after the horse has learnt to follow the bit; the curb should be used with release; by taking the rein a bit, making the horse yield, you the release the rein more and the horse follows the rein to a low frame, opposite of what most people think, it's to lower and get the nose forward, not to pull it in. To raise the head, you use the snaffle, and both curbs and snaffles can be used together.
Gags are to me most confusing..as with the curb, you feel the rein lighter than the horse does. But since there is no strap or chain to keep the bit in place and to signal for the horse to move the nose in, the mouthpiece just slides up against the horses ears when you pull the rein. I've never really understood what that's supposed to do, but please inform me if you know.. To me, this is the harshest and most confusing bit, I wouldn't use it. (Much like the ''bitless bridle'' ;)
So, what makes a bit harsh? If it doesn't fit the horses mouth and education, it's harsh. A too fat bit is harsh since it doesn't get enough place in the horses mouth, as well as a too thin if you know that you can be a bit heavy in your hands. Twisted or bumpy parts on the mouthpiece is only there to make it harsher and should be uneccessary. Copper inlays are usually there for the taste and should be o if they're smooth. Ridicculously high ports (some western bits and possibly some portugeese curbs) is very harsh. Chains or lots of joints (waterloo bit or what it's called? I've even seen a bit made of a chainsaw chain) are both harsh and confusing since it givs a weird preassure and tightens around the chin. Too many moving parts can be confusing too. A two jointed snaffle is never good with a tight noseband (tho nosebands should always be loose) since it can hit the palate and the horse can't get away from that.
Long shanks on a curb might be seen as bad, but many horses prefers tht since it gives a softer warning before the rider uses the rein. Of course it also makes the differense in feeling between hand and mouth bigger.
Other than that, there arn't much ways to saw what is good or bad, mostly it depends on what the horses like and how well it fits in that very horses mouth.
Mostly it depends on how it's used, but I guess a curb or gag in the wrong hands are worse than a snaffle in the same hands.
Don't put the curbstrap too lose, it will make the bit more like a gag than a snaffle and completely destroy the purpose of it, making it harsher and more confusing than it would if the strap was right.
On a sidenote; Bitless can be very good, but can also be put under the three main categories I''ve mentioned.
The bitless bridlr and the like (with straps that hugs the horse) is more of a gagbit.
The hackamore (mechanical) is a curb, and shouldn't be used on a green horse, even if it's well educted with bits, it should learn to seek the bitless before you use the hack.
Riding cavessons, muserolas, sidepulls and the like, are like snaffles.
I think that was all.. so basically, whatever fits your horse and your purpose is a mild bit when you use it right. :)