The difference is that in astronomy you have enough data points that you can do research to confirm or deny you hypothesis. Edwin Hubble didn't use one Cepheid variable star to prove his "standard candle" theory, nor did he use just a few galaxies to prove his expansion theory. His theories are still being tested today because there's sufficient data points out there to continue testing, and as we refine the instruments and methodologies for these studies (e.g. using solar-orbiting satellites to increase the base for the angular parallax of a star or cluster to make the "standard candle" more accurate), we continue to test, prove and refine the theories in question.
In AGW, we have ONE data point, our environment. Statistically, our measurements of the environment are pretty close to useless because of the lack of testing sites, lack of access to a lot of global locations, lack of understanding about deep-sea currents, and lack of rigor in the testing methodologies over the last couple of hundred years (they are at discussing utilizing observations from sea fairing captains back to the 1600's, I'm wondering just how accurate the instruments where then, given we're talking about variances of 1/10 of a degree).
All in all, the AGW thing reminds me more of the saccharin scare in the 80's than anything else. One place published a set of data, everybody else used that data either to tune their experiments or as the entire basis for their "studies" and nobody questioned the METHOD of the original study. As with cold fusion, saccharin was cleared after somebody tried to repeat the original study, but unfortunately (and this is bared out in the emails) the AGW folks don't share methods and data with people who haven't proven their "loyalty". THIS is why lay people don't trust the folks involved, they clearly have an agenda that supersedes their scientific rigor and it has cost us hundreds of trillions of dollars with very little to show for it.
All that said, renewable energy is a laudable goal and research into that area must continue, but the best way to manage resources is through governed self-interest, which is coincidentally the basis for capitalism. If you come up with a way to get 85% efficiency from a solar panel, you'll get all the money you need to make it happen and bring it to market without one single dime of government money being involved beyond basic research. Instead governments around the world are taking on the role of venture capitalists, investing in "ideas" with very little solid science behind them and subsidizing technologies that are not up to the demand (e.g. 19% efficient solar cells, "bio fuels" that take more energy to produce than they provide, etc.). To the lay person, this simply doesn't make any since, and it shouldn't make since to anybody, unless they are investing in the companies providing these duds (like Mr. Gore).
Finally, to anybody who knows even a little about the scientific method, the argument that a "consensus of scientists agree" on the subject of Global Warming just doesn't hold water. A consensus of scholars agreed that the Earth was the center of the universe including the greatest philosopher in history (Aristotle), despite Ptolomy presenting sound evidence to the contrary. It took 2000 years and a brave Catholic priest (Copernicus) to present indisputable evidence to the contrary and even then, it was another 450 years or so before his boss, the Pope agreed with him. CONSENSUS IS NOT SCIENCE!