Here are some quotes (to the best that I can remember them) that I've encountered in recent years:
"How can anyone do better than Google?"
"A black person won't become president of the United States within my lifetime."
"Without Albert Einstein, it would've taken another 30 years to develop general relativity."
"Without Microsoft Internet Explorer, the World Wide Web would've been set back for years."
"I don't believe we will ever create true artificial intelligence."
"The human body is so complicated, I don't see how we can dramatically extend human lifespans in the foreseeable future."
The first quote is a question that a lot of smart people have good answers for, the second is now proven to be false, and I strongly disagree (in some way) with the remainder. The third and fourth quotes deal with alternative history, while the last two express opinions about the future.
The common theme, as I see it, is a combination of pessimism and close mindedness. The two are related but not necessarily correlated. For example, one can be extremely imaginative about all the possible doomsday scenarios that might come to pass. I won't discuss first two quotes in detail (except to say that they were opinions expressed by well educated professionals) and will instead deal with the last four.
I'm not very knowledgeable about general relativity, so I won't comment on it much. But Einstein clearly collaborated with and built off the work of many others including Lorentz, Hilbert, and Riemann. But we humans tend to funnel the joint accomplishments of many people into just a few (or in this case one person). Yes, Einstein was a genius. But I highly doubt that he was smarter than the collective intelligence of all the other physicists and mathematicians working on the problem at that time. We tend to deify Einstein for the accomplishments of that era without sufficiently acknowledging all the hard work that everyone else did.
The Internet Explorer opinion seems impossibly difficult to defend. IE wasn't even the first commercial web browser. That honor falls to the Netscape Navigator. The idea that the World Wide Web would not charge full steam ahead without one software product seems a little absurd. It was already clear at that time that the internet was going to be a huge deal. Where there is a will, there is a way. Someone was going to develop a dominant browser, and it just so happened that Microsoft did it (some might argue that IE's dominance was more the result of Microsoft's market leverage rather than good design, but let's not split hairs).
In essence, we get so locked into what happened, and how monumental the end result was, that we fail to put the full process into a proper perspective. This has two effects. First, it makes us think that the world's fate (or at least various aspects of it) balances on a knife's edge -- that our average quality of life might have been much worse had a few events happened differently (this is not necessarily in conflict with ideas like the butterfly effect or chaos theory). Second, it makes us more pessimistic about the future. If we only focus on the final results and not the incremental journey we took to arrive at those results, then it might seem impossibly hard for us to make more breakthroughs in the future.
It is true that I am overly optimistic compared your average Joe. However, I would argue this is mostly due to all them Joe's not fully reflecting on our technological and scientific prowess, where we are, and how we got here. Rather than addressing the last two quotes directly (which would require a lot of speculation), I'll simply remind everyone of some recent breakthroughs.
Suppose you had asked someone in 1985 whether, in 20 years, the developed world would be connected via a global information network that transforms almost every aspect of our lives, that person will probably think you're either nuts or spent way too much time watching bad sci-fi movies. And yet, that is exactly what has happened.
Within the same time period, we have now developed a variety of methods to efficiently map the human genome, as well as perform some fairly complex gene manipulations. Again, most people back in 1985 would probably think you're nuts for imagining so much progress in such a short time.
As we continue to gather more knowledge, conceptual shifts are often required to more effectively model and understand the world around us. Unfortunately, that makes it very difficult to foresee how we should approach currently unsolved problems from a technical standpoint (if we did, then it wouldn't require research). But such transformational shifts in thinking have happened in the past (sometimes gradually and other times violently), and they will continue to occur in the future.
So I want you to ask yourself a couple simple questions. First, how long do you think it will take us to develop general purpose programs that can interactively learn and adapt to heterogeneous environments (e.g., an off-the-shelf robot that can learn to perform all sorts of different house-hold tasks)? Second, how long do you think it will take before medical advances allow us to healthily live to 120 years? Would you answer 25 years? 100 years? 500 years?
I expect most people would give a relatively small answer (perhaps in the 20-30 year range). Keep in mind that, as new technology areas mature and enable new businesses, the rate of progress often experiences a huge explosion. That's what happened with automobiles, airlines, computers, cell phones, the internet... and basically every cool new technology in the last 200 years. So it's what I think will happen with smart programs (and machines) and life extension technologies.
I'll conclude with a couple of my favorite quotes:
"The best scientist is open to experience and begins with romance - the idea that anything is possible."
-- Ray Bradbury
"If you can dream it, you can do it."
-- Walt Disney