PDDL2.1 was designed to push the envelope of what planning algorithms can do, and it has succeeded. It adds two important features: durative actions,which take time (and may have continuous effects); and objective functions for measuring the quality of plans. The concept of durative actions is flawed; and the treatment of their semantics reveals too strong an attachment to the way many contemporary planners work. Future PDDL innovators should focus on producing a clean semantics for additions to the language, and let planner implementers worry about coupling their algorithms to problems expressed in the latest version of the language.
@article{arxiv.1109.5665,
title = {PDDL2.1 - The Art of the Possible? Commentary on Fox and Long},
author = {D. McDermott},
journal= {arXiv preprint arXiv:1109.5665},
year = {2011}
}