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I gather that if there is a reasonable objection to the AGW thesis it is not that there isn't an available mechanism (greenhouse gases) that we would expect to have the result of increased global temperatures all other things being equal, but rather that we don't know if all other things are equal.  I.e. we don't know if there aren't other compensating causal factors.  Second, I gather that the objection is that we don't really know the degree to which (no pun intended) greenhouse gases would increase global temperatures, all other things being equal, and hence we can't with any high level of certainty look at the actual changes in global temperatures and say that such and such a percent is due to greenhouse gas emissions.  
 
Anti-AGW could really be any one of the following, and it is sad that they all get lumped together, since there actually is a serious debate to be had over 3-6. The political discourse unfortunately is such that on the left you are branded a heretic if you take anything on this list seriously, and on the right, you are branded a heretic if you think that there is merit to the AGW thesis at all (which 3-6 of the following do).  
1) You think that global temperatures aren't rising (these people, I gather, are simply wrong)
2) You think that temperatures are rising, but that it's not attributable to human activity in any way.  (again, I gather that there is little disagreement amongst scientists that these people are also wrong).
3) You think that temperatures are rising, and that it is partially attributable to human activities, but that the percentage attributable to human activity might be substantially lower than we have been led to believe by Gore, for instance.
4) You think that temperature is rising, and that it is largely attributable to human activities, but that the expected effects of increased temperatures are not likely to be as dire as we have been led to believe by Gore, for instance.
5) You think that the temperature is rising, and that it is largely attributable to human activities, and that the expected effects of increased temperatures will be dire, but that stopping greenhouse gas emissions will  be very expensive, and have similarly dire consequences for the economy.  A cost benefit analysis of the situation would favor not accepting substantial limits on greenhouse gas emissions. 
6) (a variation on 5, I think) You think that the temperature is rising, and that it is largely attributable to human activities, and that the expected effects of increased temperatures will be dire, but you think it is too late to do anything meaningful about it.
 
As an added wrinkle, even if I granted that from the perspective of pure science, your analysis would be right Matt, the worry is that this is a decision under uncertainty with potentially dire consequences if we get things wrong.  If someone said to you "given our current best evidence we think there is about a 30% chance that you have cancer that will kill you in 6 months if untreated.  If treated you will have a 90% chance of survival. You have two options.  1) you could do a range of other tests that will take 5 months, after which our certainty as to whether you have the cancer will approach 90%.  If you do have the cancer, though, at that point you will almost certainly die.  2) We can treat you, hence giving you a 90% survival chance, but the treatment is very expensive, and you will end up paying us 30% percent of your income for the next 20 years."  Which would you do?  I'm not claiming that the answer to this question is obvious.  Perhaps you don't even agree that this is even (roughly) analogous to the decision we do in fact face vis a vis global climate change.
 
In any case, I'm curious whether you'd agree with this appraisal of things.