The 3rd and 4th International Planning Competitions have enriched the set of benchmarks for classical propositional planning by a number of novel and interesting planning domains. Although they are commonly used for the purpose of evaluating planner performance, the planning domains themselves have not yet been studied in depth. In this contribution, we prove complexity results for the decision problems related to finding some plan, finding an optimal sequential plan, and finding an optimal parallel plan in these planning domains. Our results also provide some insights into the question why planning is hard for some of the domains under consideration.