Explain plans are pretty complicated to read. Figuring out which portion of the plan is causing the main performance issues is not necessarily obvious. A few tools have sprung up to help with analyzing them, making it easier to identify the slow portions.
Explain analysis tools
Visual explain
Complicated query plans can be difficult to read, with only the indentation level suggesting how nodes that fit into one another are connected. One way to help visualize complicated plans is to graph them using visual explain, a feature available in the pgAdmin III tool: http://www.pgadmin.org/.
Seeing how the nodes fit together for the more complicated structures like multilevel joins is extremely valuable for learning how they work...
