Rapid change is bad. It is bad for all who live through it. Whenever conditions change faster than organisms can adapt it is generally bad for all, even those that can. We can adapt, nobody really doubts we can move things around and continue. But most all the other species cannot. Does anyone really want a world with seas filled only with jellyfish, with forests home only to raccoons and pigeons living off our scraps? Humans will survive, but they won't be happy. Our adaptation may require our civilization taking a few steps backwards. I'd like to avoid that world.

If you consider Nature in general, nothing is good or bad. Colder is better for organisms that prefer cold, warmer for those that prefer warm. No life at all might be best for the geological (or should I say areological) wonders on Mars.

Ethically speaking, I think most climate change "alarmists" (what is the equivalent word with positive connotation?) are either worried for the species that will go extinct, or the poorer humans that will suffer the most from it.

The majority of the population lives near the coast, and this percentage is increasing.

Those people (billions of them) will either be displaced, resulting in a huge humanitarian crisis, or spend enormous amounts of money and effort to create dykes and levies. Remember Hurricane Katrina's effect on New Orleans? If the world went the latter direction, such disasters would be a risk for every coastal city. And climate change will likely involve more frequent and intense storms...

There is a temperature range in which organic life flourishes the most. Too cold, chemical reactions slow down to a crawl or stop. Too hot, organic compounds break up. Somewhere in between is the sweet spot. So a warmer planet relative to now doesn’t have to be bad. Maybe Earth would even get closer to the sweet spot, from a global perspective.

The problem is the transition speed to the new global average temperature. 4°C increase over a million years: Organisms have time to evolve and to adapt to the new conditions. The same change in 100 to 200 years: Many species will suffer, especially ones occupying ecological niches.

Regarding humans: Generally speaking, existing infrastructure is not equipped to handle the extremes that occur more often when Earth is warmer, e.g. rains can cause more floods because warmer air transports more water.

Good and bad.

Setting aside the changing weather patterns (also good and bad), a warmer planet will likely be good for some kinds of life and bad for others.

It will be good, particularly, for vegetation (discussed in this article, the greening of the Siberian tundra). It will also be good for animals which depend on the now more plentiful vegetation and opened up territory. It will be bad for animals that are dependent on the old climate. For humans (ignoring ethics and other concerns over species becoming extinct), it's probably a net positive as this means more arable land and territory as well. Though this may only be temporary.

At sea, it's again a mixed bag. Cold-blooded animals, in particular, are very sensitive to changes in temperature. Most fish and sea creatures are also sensitive to levels of CO2, O2, and acidity. Increasing temperatures and CO2, even if still technically sufficient levels of O2, may render some species infertile or unable to thrive. Increased CO2 and reduced O2 will make large swaths of the ocean uninhabitable by fish and other creatures. The CO2 will help CO2 consuming ocean life, but until a new balance is reached O2 consuming life will likely be greatly harmed. This has major ramifications for people dependent on the ocean for their sustenance or livelihood.

Going back to the tundra and its recent browning described in the article, it's possible (note: totally my hypothesis, not based on any research, this is just a thought experiment based on systems thinking and modeling) that the initial greening was due to abundant local water (frozen in the ground) paired with increasing temperatures. The plants spread, temperatures rose more, creating a positive feedback cycle. However, this also results in less snowfall (or retained snow) meaning the water may not be replenishing at a sufficient rate to sustain the increased local demand by plants. In turn, plants die (the recent browning event). But a browning event doesn't imply that the temperatures will or are dropping. So it's not clear if this will be cyclical, or if it is at what period. Perhaps a new equilibrium will be reached, or new, less demanding, plants will take over. This also assumes something of a closed system, the real world is far more complex than this simple model.

This could be very similar to any other boom/bust cycle in nature or economics. Major initial change causes immediate significant positive results, followed by negative, followed by ??? (neutral, more cycles, who knows yet).

Absolutely, yes it is great for planet wide life and overall diversification, typically at the expense of the incumbents.

It's bad when modern society depends on the integrity of the biosphere.

Define "bad".

The question as regards to human habitation is: will a warmer planet be a planet that can support us as well, will it be a planet that we enjoy living on to the same degree?

There's more to it than "warmer".

Canada and Siberia will be great for wheat. Plan ahead :)

The problem is when it warms as fast as it has.

famine is bad for most species