I ended up visiting anyway (I really wanted to research Philly Cheesesteaks, Maine Lobster and see the colours of the New England Autumn/Fall), but I definitely thought long and hard about it and was apprehensive.
I also made sure my medical travel insurance was solid.
Now, maybe I live in a paranoid white-male bubble that reads about this stuff, and that informed my apprehension, but I personally think it would turn off many, many people from visiting.
That's only true for countries under the U.S. Visa Waiver, which are maybe 5% of the world's population (basically European Union and Australia).
For the remaining 93% of us, we have to think long and hard and plan far ahead before visiting the U.S., and that means being aware of all the issues or risk deportation.
http://en.wikipedia.org/wiki/Visa_Waiver_Program
And yes, I believe it does affect conversion significantly, but, coincidentally, we're the poorer 93% .
"Following the September 11 terrorist attacks, the George W. Bush Administration decided to tighten entry requirements into the United States"
I even was a few meters from the U.S. (Niagara Falls) and avoided going because of all the visa complications and expenses plus the security measures (if it had been a given I'd be accepted I would have gone, but they don't like young unattached people saying they want to do "tourism", they expect we'll want to stay).
It wasn't the case in SFO, after a 12 hours flight from Paris, the agent was OK but far from welcoming.
I have to apply for a visa, and at the time there was a significant chance of it being denied - and forget about being refunded, and chances of it being accepted after re-applying drop dramatically. So it isn't worth it.
I could probably be accepted for a visa now since I "settled down".
And Canadian customs and inmigration were very friendly, I don't doubt the U.S. people might be the same (though they did fingerprint my father the time he was there, and he reported a few not so friendly times, and he is a frequent visitor).
Europe had terrorist incidents involving planes long before 9/11. As a consequence air travel (domestic or international) had greater security checks but nowhere near the level we see now in America.
Here is the problem, no amount of security checks can guarantee that there will not be a terrorist incident on a plane.
Even accounting for 9/11, other terrorist incidents on airplanes and plane crashes, the likelihood of you being killed in one is very small.
But the politicians are faced with a decision, respect civil liberties and relax security to common sense levels or ratchet up security to ridiculous levels. If they do the first and a terrorist attack occurs they will get the blame.
Ultimately it is a risk problem, and it is the voters that need to tell those in charge that they understand the risk and won't blame the government for "not protecting them" if something bad happens to them.
It has been reported here in Australia that the US tourism bureau (or whatever they are called) is very concerned about it. Numbers are way down over the last several years. They will be running an ad campaign this year
Airport security stuff annoys me to no end in other places too because half of them, or more than half, seem to literally be standing around doing nothing and the other half do their jobs rudely.