At least a few times each week I find myself talking people out of filing a patent. Here are the most common reasons why I don't think a patent should be filed.
Unpatentable Subject Matter
Not every single invention or business idea can qualify for a patent. You may have the best idea in the world but the fact is some things just cannot be patented. In general, patents can only be filed on new inventions that are objects, machines, or complex methods such as a new method of isolating a chemical or a complex software process. So if your invention is a new way to sell cars online or a new way to teach children how to read, it will not fall into one of these categories. It may be a great idea and you may make millions, but, you most likely will not be able to get a patent allowed. Things like food recipes, smartphone apps, and games are also common inventions that I see and they are difficult because sometimes they fall into "patentable subject matter" so in theory they should qualify for a patent ,but, they usually run into novelty or obviousness rejections (see below).
The invention is not new or it is obvious
Only new and non-obvious inventions can get a patent. Another common problem that I see is an inventor will spend days or weeks searching other patents online and approach me with their new invention and tell me its new because they did not see any other patents out there on their idea. Here is the problem, if the patent office can find out that ANYONE ANYWHERE around the world created your invention before you they can reject your patent. In other words, don't limit your search to just patents. Many times I have spent less than 5 minutes searching google and have been able to find an invention nearly identical to the one the inventor has approached me with even though they had spent hours (and sometimes days) searching themselves. Their mistake was only searching for patents and not doing a general google search to see if anyone else had invented the same thing before them.
You want to keep your invention "top secret"
While it is a good idea to try and keep your invention secret for as long as you can, many inventors do not realize that patents and patent applications are published online. There are no secrets with the patent office or the public. There are some inventions that may be better off kept secret. An example may be a food or drink recipe (think Coca Cola) or internal processes that you don't want your competition to ever know about (for example search engine algorithms). In those cases you may be better off keeping your idea as a trade secret and not spending money on a patent.
You don't have a plan
Patents are expensive. I usually tell people you better have a good business plan before you decide to spend a lot of money on a patent. Sadly there are many inventors who think that filing a patent is all they need to get rich. There is a whole industry setup to take money from naive inventors (have you see any of those commercials on TV?). Many people see filing a patent like buying a lottery ticket that is sure to pay off and that companies will eagerly pay them for their patent application. Sorry to break it to you but this is probably not going to happen. It is extremely rare for a company to buy or license a patent application from an inventor without that inventor first establishing a business model around that invention. In fact, something like 95% of all patents filed never make money for the inventor or company who filed them. I usually see this issue arise mostly in the software space. For example, Google is probably not going to care if you have a patent on a new app to share photos with your friends (they probably have 1000s of similar patents themselves). However, if you can demonstrate that people love your app and that millions of people are using it then they may want to buy you out (not because you have a patent but because you have a patent AND have demonstrated that the concept works).
Still think you want to file a patent? Check out our post on the typical invention to patent timeline.