As other reviewers suggested, it's remarkable how an ER season can be made or broken by good guest appearances. This is a shortened season, cut to 19 episodes by the writer's strike. The first nine episodes are dominated by the guest appearance of Stanley Tucci as the new head of the ER, having been transferred from intensive care. In episode 9, Skye Wexler, another major guest star, takes over as temporary head of emergency medicine, and Dr. Pratt considers leaving County General.
After Tucci, the most important non-lead role is Leland Orser as Dr. Lucien Dubenko as chief of surgery. His presence is usually the only thing which makes Parminder Nagra's role as Dr. Neela Rasgotra interesting. She is one of the very few major characters who, for me, tend to add to the mix of characters and plot.
Goran Visnjic and Maura Tierney are still the leads, but their roles are less important to the main action. The addition of David Lyons as Dr. Simon Brenner spices things up around episode 14.
John Stamos' role grows in importance. But the biggest change with lead characters is Dr. Archie Morris' becoming a far more serious doctor, comparable to Pratt and Kovac as the ER's best attendings, even if, sometimes, he still appears to be a klutz.
Near the end of the season, Hal Holbrook appears as a guest. That is always a treat.
One of my constant aggravations on ER is that whenever one of the doctors or nurses shout's "Call Security", it seems as if Security never arrives, until the doctors, nurses, and clerks have already subdued the disturbance. One would think that the ER is one place where two security guards would be posted on a 24 hour basis. This season has just such an episode. It seems as if most in hospital scuffles could be put down in a few seconds if the guards were a few steps away.
The last episode of the season ends with a great cliff-hanger, and an appearance by one of my favorite actors, Steve Buscemi.