I noticed that many sites in my field have incredibly similar terms of service. Is it illegal to copy portions of the terms of service from another site?
I would not copy them verbatim. Remember, if the ToS you copy from is not legally accurate, especially in your jurisdiction, you have nothing. You can put anything down in a contract that you like, but if it is not legally accurate, the whole contract can be voided in many cases.
Bear in mind that a ToS will technically be copyrighted too, so copying one verbatim is not a good idea.
Correct. Looking at other examples is a great way to get a grasp on the situation. However, you still need to know the law. Having a lawyer look over your creation or having a lawyer draw this up for you is best. That said, not all lawyers where created equal. Many contracts have been pierced because they were not written well.
After all, do you really care if your TOS are copied? It's not contents, and lawyers would tend to look at others too. Maybe add this, subtract that, but it's more or less the same story in most countries anyway. I can't see a lawsuit going very far for copying TOS in most cases at least.
You would be surprised. Your best bet is to modify the text some and to put it in a different, but still proper, order. If you paid a lawyer $1000 for your ToS, would you mind if others took it for free?
It's best to write your own. If you copy and paste someone else's TOS without even reading it... What's the point? You should know and set your own rules.