First, is the planet getting warmer? On that I'd say there's general agreement, although it is not a 100% consensus.
It's a 99.something% consensus, which is as solid as any consensus among a large population is ever going to get. Out of 13,950 peer-reviewed climate articles from 1991-2012, only 24 reject global warming. (source)
Second, if it is getting warmer, is it caused in large part by human activity or is it part of some natural variation? This is the sticking point. If it's part of a natural variation in temperature -- and I will point out many such variations have happened in the past few million years, all without any input from humans -- then there is no need for us to radically alter our life to stop it because such actions will have no positive climatic effect while having a signficant negative effect on quality of life.
All the evidence we have for previous natural variations show them to be slow (or extremely rapid, as in catastrophically rapid - impact events or super-volcano eruptions); the changes we're seeing today is way too rapid to conform to any known natural cycle. The difference, of course, is that we're around and actively adding greenhouse gasses to the atmosphere. In short, not a "natural variation".
Third, if it is anthropogenic, what should we do about it? Curtainling greenhouse emissions is an obvious choice, but is it the best one? How severe are the predicted warming effects? The economic and socio-political upheavals from drastic policy changes might be worse than adapting to a changing climate. And how much confidence can we have in the predictions regardless of how severe (or not) they may be?
We don't know; that's the problem. We don't have any crystal balls, so we don't know what the most effective strategy is, or exactly how severe the effects will be. What we do know is that large climate changes historically have been responsible for some of the most drastic extinction events we know of. And it's pretty easy to speculate about what a massive dying-off of e.g. marine life would do to coastal communities - as is the effect on the same communities of rising sea levels.
These are not minor issues. They deserve to be studied and debated *in depth* before drastic action is take, if for no other reason than to determine that we're taking the *most effective* action possible. This whole "the debate is settle and if you don't agree with us you're a denier" smacks of the same kind of thinking that gave us an Earth-centric cosmic model and burned "deniers" as heretics.
No, these are not minor issues, and the ramifications of the decisions are huge. In the end though, doing nothing is probably the worst decision; there is a tipping point somewhere (the edge of the cliff, so to speak) which going past that there is no turning back. More research and discussion is always welcome, but that should not and cannot stop us from starting to act - if nothing else to slow down the rate at which we're approaching that tipping point.
The analogy with the earth-centric cosmic models and burning of a few heretics is really stretching it when we're talking about the possibility of mass extinctions of not only humans but a lot of other species as well.
The earth will survive, and life itself will survive. The question is, will we? And even if we do, in what kind of society? One that has planned for such an eventuality, or one that has had to just react to it. One is liveable, the other is a post-apocalypse society; I know which one I'd rather (have my kids) live in.