Does one degree even have that much of an impact on anything? I know I can barely tell the difference between say, 70 and 72 (Fahrenheit, so about 1 degree C).
How do you feel when you're at 100 F and when you're at 102 F? It's a serious analogy, actually. And the 1.5 degree change is IF we follow a best-case scenario of mitigating carbon, which we're nowhere close to doing. If you knew that you'd go to 102 F, you'd be distressed at the people who caused it to happen. If you knew that going to 102 F was a best-case scenario, you'd be worried. Especially when people were pshawing it.
Ecosystems are alive and integrated systems. Whether or not a specific plant or animal can withstand a change in temperature is unimportant. There're temperature thresholds that can change the breeding cycles of animals, or disrupt the breeding between the sexes. There're temperature cycles than change the rate at which organisms mature. There're temperature thresholds that change feeding and migration patterns. The difference between mass death of coral reefs is a couple of degrees, one bad El Nino on top of pre-warmed water can cause a wipeout.
AGW is going to cause changes in precipitation patterns and species migrations. Precipitation patterns are important, because we've co-opted so much land, and all the current borders are drawn with current climate factored in. Species migrations are important, too, because there's the strong possibility that AGW will cause migrations that species will be unable to perform.
A robust ecosystem can survive a few shocks. It can even weather quite a few of them. The problem is that we're hammering over and over. AGW is one thing. Land co-option is another. Changing ocean pH is another. Overharvesting and deforestation is even another. For goodness sake, we don't feed certain seafoods to pregnant women anymore, because of the risk of neurological damage.
AGW is not a red herring. Even if we're just talking about humans, it's going to be a factor in disrupting global politics as the climates shift irrespective of international borders. And the long-term justice issues of the rich nations polluting into the Commons is another issue of international stress.
And that's without mass extinctions, which are sufficiently likely to cause a runaway effect, that it's actually worth worrying about. Like I said, it's like bankruptcies. Once things start to unravel, you might not have the resources required to fix it. Even worse, there's future decay built into the system, so even when you notice the problem and decide to stop, you still have to live with what you've done.