When does an idea count as intellectual property in a game? Put another way, when am I illegaly copying an idea?

There's one particular example that I'm curious about:

I really like the idea of jumping below a block and hitting it with your head to make some sort of bonus item sprout from it. Now this is directly what is done in Mario. Am I allowed to do this in my game or would I be stealing their intellectual property?

Ideas are not a form of intellectual property. Complex processes and mechanics can, in some cases, be protected by patent. But a simple game design mechanic like the one you are describing cannot be protected under any IP system I know of.

It's possible that a combination of a certain mechanic along with certain visuals might combine to form a violation of 'trade dress' laws if your game was so close to another in appearance that it could be argued that you are likely to confuse potential customers into thinking they are playing someone else's game. But merely "jumping below a block and hitting it with your head to make some sort of bonus item sprout from it" is not enough for this to be the case.

I really like the idea of jumping below a block and hitting it with your head to make some sort of
bonus item sprout from it. Now this is directly what is done in Mario. Am I allowed to do this in my
game or would I be stealing their intellectual property?

Nintendo's IP is Mario's name, Mario's look. Nintendo owns the copyright of Mario's look, and Nintendo
owns the trademark of Mario's name. But the "idea of jumping below a block and hitting it with your
[character's] head to make some sort of bonus item sprout from it" is not protected. People will know
you got the idea from Mario games, but Nintendo can't sue you for using it.

You should read up on copyright, trademark, and patent. Google those terms, and read this forum's
FAQs, and read these:
http://www.sloperama.com/advice/lesson39.htm
http://www.sloperama.com/advice/faq61.htm

Here is a simple break down:

Copyright - Original works of authorship fixed in any tangible medium of expression. Tangible being a very inclusive term (basically anything that can communicate the work is tangible). Basically to for copyright to be valid it just needs to be original. Words and short phrases are not copyrightable. No need to register but it helps

Patent - Any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement. This one is the most protective, but shortest right (still 20 years). This one will can trump copyright at times, so if you make something useful and artistic (like a cool looking bike rack) you may be able to only get a patent if it is more useful than artistic. Have to register to get protection

Trademark - This is actually a general term that includes trademarks, service marks, certification marks, collective marks, and trade dress. These include any word, name, symbol, or device, or any combination that you use with a bona fide intent to use it in commerce. Have to register to get protection.