In spite of being a marketing spin and link bait, Heroku article on microservices does a pretty good job at explaining why microservices matter and, at a very high level, what challenges they pose.

However, microservices aren’t a silver bullet, and they won’t make a sluggish IT organization fast. While individual services become more robust and less complex, the overall system takes on the many challenges of distributed systems at the network level. Despite their challenges, they’re here to stay because they map better than anything else to the software landscape of the future: parallel development, platform-as-a-service deployment, and ubiquitous use.