1.  GET RICH QUICK - Patents are expensive.  Realistically, for even a relatively simple invention, filing a utility patent application (not to be confused with a design patent application) is going to cost at least $10k.  The costs don't end there.  18 months after filing the application will publish and the prosecution phase begins.  Prosecution involves arguing with the US Patent and Trademark Office (USPTO) and typically making very calculated changes to the application, in order to convince the  USPTO to grant the application and issue a patent.  There are government fees along the way as well as maintenance fees charged afterwards to keep the patent rights "alive."  After all is said and done, $30k - $50k is not an unrealistic number for a patent that will afford the owner solid protection.

2.  NAME IN LIGHTS (or at least on a plaque) - See point number 1.  An issued patent does give an inventor credibility and, perhaps, bragging rights, but there are much cheaper ways to boost an ego.  Furthermore, the plaques cost extra.

3.  YOUR INVENTION IS COOL, BUT ...  -  Again, see point number 1.  Ask yourself whether your invention addresses a specific need that is faced by enough people to create a market.  Is there another tried and true solution that may not be as cool as your idea, but still works?  Are you willing to put the time and money in to developing the invention into an actual product?

4.  YOU WON'T ENFORCE IT - Patent prosecution is expensive, but pales in comparison to the expense of enforcing a patent.  Enforcing a patent involves determining whether a competitor's product fits within the scope of at least one claim of your patent; contacting the competitor to warn them that there may be an infringement issue, an accusation  they will vehemently deny; months of lawyer chest-thumping; and then, perhaps, filing a lawsuit.  If the suit proceeds to trial, the costs can reach seven figures very quickly.