I'm definitely among the ranks of the techno-optimists. We get used to it all so quickly, but if we were transported back just a few decades ago - or if we took someone from that time and brought them to the present - we'd see pretty quickly just how wonderful progress has been.
Anything that can be done according to the laws of physics will eventually be done, but we do have to be careful about catastrophic risks (nuclear war, supervolcanos, asteroids, super-plagues, etc). We really don't spend enough resources to prevent these, considering just how big their impact would be.
There's a great book about this compiled by an Oxford University polymath (Nick Bostrom):
http://www.global-catastrophic-risks.com/
http://www.amazon.com/Global-Catastrophic-Risks-Nick-Bostrom/dp/0199606501/