This is a list of guidelines that I created a while back. They are meant to describe what constitutes as godmodding, primarily during a T1 based RP fight, and can be used for virtually any setting or genre. This is just for those who may not be experienced with RP combat and might need a little help.
1: Teleportation of ANY kind, including portals and moving at a speed that is so fast that it might as well be teleporting, if used during a fight or for the purpose of starting a fight, such as teleporting behind someone to hit them or anything like that, is a god mod. It is a mod because it makes it nearly impossible, if not entirely impossible to hit you. It is ok to use outside of combat, but once your character is engaged in a fight, it becomes unfair to use.
2: You cannot block an attack with your body and take no damage from it. No matter how insanely powerful your character may be, if you are directly hit with any attack, you take damage from it that is relative to the attack dealt. For example: if someone slices your arm, and you fail to either dodge it or block it with a weapon or shield or something of the like, your arm gets cut. Depending on how the attacker worded the post, your arm could in fact be cut clean off.
3: Your attack, no matter how complex, no matter how powerful, no matter how fucking long it took you to charge it, CAN AND MUST be able to be completely avoided in some way. Now the way it can be avoided can be extremely hard to figure out, sure. Or it can take a lot of power to avoid, power equal to the amount of power it took to make the attack. Which means that if you put every last ounce of your energy into an attack and someone manages to avoid getting hit by it, you're shit out of luck because you just used up all your power and are now almost completely defenseless. Bottom line is, there is no such thing as an attack that cannot be avoided, period.
4: Armor does not protect you from everything. If someone with the strength to punch a building and turn it into a crater slashes you with a sword, I don't care how thick your suit of armor is, that steel isn't going to prevent it from penetrating. Armor is almost completely useless in a high power RP fight, and it is recommended that it be used only for show. However this is limited to high power fights, as stated, and armor can be very useful in the right scenarios against the right opponents. RP genre and setting should also be taken into account. However the base rule always applies: Armor can protect, but not from everything.
5: In case I didn't clear this up earlier, an attack requires an equal amount of power to create as the amount of damage it causes. So the harder it is to defend against, the more power it must drain from you in the process. For example: If someone makes an attack that is powerful enough to take out several people and deal incredible destruction to the area at the same time, that is what we call a "Finisher" move. Such a move usually requires the user to be drained of nearly all power after usage. It is a trump card, if you will.
6: Illusions are a tricky thing to call into an RP fight for several reasons. One of those reasons is that it's so easy to god mod when using one, and usually takes a lot of skill and practice in order to use one that is not a mod. In order to use an illusion, you MUST make it possible to whomever it is being used on to figure out that it is an illusion. You can make it difficult to figure it out, but it must be able to be figured out without asking if it is an illusion. Another, more noobish way of doing it, is to include in the post itself that it is an illusion. You can't make some kind of super crazy attack that nearly destroys someone and then say "oh it was an illusion, don't worry" UNLESS you make it so that they can figure out what it is BEFORE it is used. They don't HAVE to actually figure it out, but you have to provide what is needed in order for them to be able to figure it out.
7: Any weapon can be broken if it is put under enough strain. You can augment your weapon, enchant it, forge it, whateverthefuckyouwant it to make it stronger both physically and magically, but it is by no means completely indestructible. Even if a weapon can be disassembled and reconstructed, or shattered and reconstructed, it can be permanently disabled/broken somehow. Again, the way to do it can be hard to figure out, but it can be done.
8: Doing something as incredible as moving planets, stars, controlling the flow of souls, or anything that outrageous, is a godmod. Doing things like that would require VAST amounts of power to accomplish even in a small way. So much power that, short of being a god character, you shouldn't be able to do. There are times when something like this may be possible, but only for story reasons, and almost never during a fight.
9: You do not have an unlimited amount of energy. You do not have the ability to completely drain your energy in an attack, and then absorb energy from something else to regenerate all the energy you used up. You may be able to become stronger or obtain more energy based on certain circumstances, but it is not unlimited no matter what.
10: Hybrid races are a tricky thing to use. But here's the deal. If you choose to play as a hybrid character that is made up of more than 1 race, such as a vampire/demon, or any other mixture, then you take into account the weaknesses that both races have. You cannot have all of the strengths and none of the weaknesses; it just doesn't work that way. Any strengths that you get from one side will be halved since you are only 50% of that race. However, the same goes for its weaknesses. You will only be half as affected by whatever that race is weak to. If you play a character that is made up of 2 races that both share similar strengths and/or weaknesses, then those would amplified accordingly, however it is generally accepted that you would never be as powerful as a pure breed. You could be close, and is certain cases could surpass a purebreed, but it varies depending on the combination.
11: Custom races/species are rarely used, but if you think you've got a great idea for one in mind and want to use it, there are a few simple rules you need to know. The race, like all others, must have equal but opposite strengths and weaknesses. There is no such thing as a perfect race or species. For every abnormal positive, there must be a corresponding negative. Follow this rule and you should have no problem creating a believable and usable new custom race.
12: Custom magic follows very similar rules as custom races. If you wish to create a new power through magic, it must have disadvantages to balance its advantages. For example, I myself created a type of magic I call Chaos magic/energy by taking light and dark energy and forcing them together. The result is a highly unstable and violent new energy that is a result of the 2 opposite conflicting energies fighting against each other. It is a very powerful form of magic and can be used in a variety of ways, but due to its unstable nature, it is almost as dangerous to the user as it is to its target. But that is the tradeoff. I get to use a much more powerful magic against my opponents, but in return, I take damage from it as well. So it works.
I hope that these guidelines will help everyone better understand the do's and don'ts of RP combat. These can generally be used in any RP type/genre, but are mostly confined to T1 style combat, which is the most used style in the world, and typically the only one used in a forum setting. If you have any questions, feel free to ask.