It's an expensive annoying long term problem.

The biggest problem in the extreme long term (hundreds to thousands of years) is the ice caps melting; we aren't sure exactly how long it will take, but it will take a long time (present models suggest we will likely see less than 1 meter of sea level rise in 2100). In the long run, having much higher sea levels (tens of meters higher than present) will be the largest negative consequence due to loss of currently desirable land area.

It will cause a net increase in arable regions and overall increase precipitation, but some areas will get drier and undergo desertification due to a higher evaporation rate and shifts in rainfall patterns. The increase in precipitation will also likely cause increased flooding in some areas, while the increased rate of evaporation and higher temperatures may cause more wildfires.

It will make winters more mild, but it will make summers hotter as well. Ironically, this will probably cause a net decrease in human deaths due to inclement weather; winters kill more people than summers do, and with the continued proliferation of air conditioning, that's likely to get even more extreme. However, it might cause some pests to expand their regions poleward.