One reason for optimism is that this time around cost, and technology, have gotten a lot more thought. Nuclear propulsion is at the forefront this time - back then, it was a political non-starter. It's possible to go to Mars using chemical rockets alone, but just barely. Using nuclear space propulsion - where a reactor heats gases to form high-speed exhaust rather than using chemical explosions to do so - cuts travel times from six months to two, and, because of better specific impulse (efficiency), allows for higher payloads. (There are no plans, as far as I know, to use Orion-style nuclear-explosive propulsion, of the sort I've written about href=http://techcentralstation.com/1051/defensewrapper.jsp?PID=1051-350&CID=1051-091102C>here, and here. Should I turn out to be wrong about this, it will probably be a sign that somebody somewhere is very worried about something.)