Tattoos are OK, as long as they are not going to cause any undue attention. A tattoo that is tastefully done won't be a problem at all.
Of course, tattoos that are similar to the one Charles Manson has, are unsuitable.
Long hair must be tied or clipped back. As long as it does not interfere with your training, or anyone else's that's fine.
Someone having a mohawk cut is no different than if someone simply shaved his head.
Fingernails and toenails must be kept to a reasonable length. The ladies are allowed a bit more leeway when it comes to fingernails, though. Nails should never be so long as to interfere with making a tight fist, or present a scratching hazard.
Glasses are only worn during non-contact times. If someone wants to spar, he must use either sporting goggles, soft contact lenses (no rigid ones allowed), or remove the glasses entirely.
Internal piercings, or ones that are securely covered, are allowed. The way I figure, a tongue piercing isn't going to affect the wearer or the partners. For other, more sensitive areas, if someone can cover it with a cup and supporter, etc., then good for them.
Piercings that are exposed, though, can pose a problem. You basically have a few options, though:
1) Remove it during your training. This is no different than any other piece of jewelry.
2) Tape it down securely. You don't want your partner cutting up his own hands if he makes an impact there. It's your own fault if you get hit there and have to deal with the consequences. Much better to remove them before class starts.
I'm not about to tell someone not to punch you in the chest during kumite, just because you have a piercing there. You chose to put it there, and the chest is a valid target. The good of the class comes before an individual student's vanity.
