OK sorry but I am so going to be sooo boring and sound like someone's Mum here but, uh, I guess I am ... so here goes.
It's not like any other piercing because it is in your mouth.
To start 86 pinkle is right -- having that metal in your mouth will damage tooth enamel, esp on the backs/edges of your upper incisors (the ones at the front).
Every time you speak, think about where the middle of the front of your tongue goes. Try and say 'this' without putting the middle of your tongue under the top front teeth. Then think about how many times a day you do that.
That means you will in the future be very likely to need crowns, which are expensive and need to be repaired (not covered by NHS). Apparently the plastic ones do this too. You may also lose some gum tissue and your teeth can fall out. Or breathe in the jewellery and need surgery to remove it.
But that is when it's healed. The scary stuff is while it's healing.
If you want to do it ... do it with your eyes open. The nasty complications of a tongue piercing going wrong are not that common but they are really, really nasty. And the damage is often permanent. Don't just ask us, ask a dentist. By all means absolutely read this.
Mine did tell me a grim tale about a recent case where the bar got embedded and had to be dug out in hospital under a general along with a bit of her tongue. Nice.
Your mouth is full of bacterial nasties -- think about your breath in the morning? The problem is that the wound from a piercing is so deep and complete that the takes a long time to heal. During that time you basically have an open wound, bathed in bacteria-rich saliva and with food getting lodged in there.
It's in a part of your body which is rich in blood vessels to carry all that far and wide. At best there is a substantial risk of infection. At worst, it carries a risk of systemic infection [blood poinsoning, brain abcesses anyone?] and deaths have been reported.
But hey that probably won't happen