Humanity has picked all the low hanging fruits of technology. Penicillin was discovered by accident exuding from a common mould. Radioactivity was discovered by observing phosphorescent rocks. Rockets were essentially an engineering problem. The elements of electricity were discovered by men using scientific instruments they could build themselves.
The rate of discovery has not necessarily slowed, but i contend that it has become more difficult to discover and propagate new things. The cost to fundamental discovery ratio has risen considerably. People are no less intelligent then they were 20,50, 100 years ago, but to find something new takes considerably more man hours and investment. Take the memristor for example, a recent discovery, just reading the course of its discovery will help anyone understand why we don't have flying cars, teleporters and intelligent robots: http://en.wikipedia.org/wiki/Memristor. The Genome project is another good example of the scale needed to make important discoveries nowadays. Just ask anyone in medical research "what's taking so long" and you'll immediately understand the scale of the problem.
There's also the cultural issues. The public is disinterested in science, roughly 50% of Americans don't believe in evolution, Only 53% of adults know how long it takes for the Earth to revolve around the Sun!. http://www.sciencedaily.com/releases/2009/03/090312115133.htm . There has also been a considerable shift since the 1970's away from basic research which is now seen as long term and unprofitable, when shareholders must have their profits now! http://science.slashdot.org/article.pl?sid=08/08/28/209220 http://science.slashdot.org/story/09/08/30/1512213/Where-Have-You-Gone-Bell-Labs?from=rss
But let us not repeat the mistakes of past generations. A English scientist, I forget the name, once said at the turn of the 20th century that everything that can been discovered has been discovered, and all that was needed now was simply to dot the i's and cross the t's. How wrong he was.
Lets also not be ignorant of the power of the technology we hold in our hands now. For a price a person can own a phone that can take an incredibly clear image, tag it with the exact location on earth where it was taken and send to anyone anywhere reasonable speaking. A MRI machine can take incredible images inside the body without a single cut. Many of us take what we have for granted. Some might remember: everything is amazing, nobody is happy: http://www.youtube.com/watch?v=WXStPqhLmIk
The challenge now is propagation. Clean water, abundant food and shelter are still discoveries waiting to happen for a large portion of our species. Flying cars, e-ink newspapers and quantum computers are no concern to the child looking for food in a rubbish tip in Africa.
Still... many nerds hold onto hopes of a Singularity. Perhaps not to the scale many of its proponents imagine, but a AI may be able to deal with relationships and datasets that human brains simply cannot understand or have the patience for. Infact as slashdot covered earlier its already here: http://www.findmysoft.com/news/Artificial-Intelligence-Robot-Scientist-Adam/