How do I tell which baseline profile my H.264 video is?

I'm trying to find out which baseline profile it's using. How do I find out this information? (The container is .mov.)... Also, how do I determine whether the audio track is low complexity? The inspector shows AAC at 44.100 kHz.

Try VideoSpec

(iTunes will also tell you if AAC is LC or not in the Info window.)

Just curious, are there any QuickLook plugins that provide the same information?

Not sure what you are trying to ask here. The files (and their specifications) are the same whether you are viewing them in QT Player, iTunes, QuickLook, Front Row, etc. Believe all of these applications are using the embedded QT structure routines for viewing although each may be programmed to report different specification elements. VideoSpec simply contains its own routines garnered from a number of different sources to read the desired specifications from the file itself.

I just meant if there were any QuickLook plugins that show the file information that VideoSpec does, so that I don't have to open an application to inspect a file's info.

QuickLook is a program/utility for viewing documents without having to open the "creator" application or alternative application. It does not appear to have any built-in "info" functions whatever over and above that provided in the finder info window or other calling application (e.g., Mail) nor do I believe it has any capability for the use of any sort of plug-in. VideoSpec is a utility whose sole purpose is the reading, analyzing, and reporting of specific information regarding the audio and video content of QT capable file just as Dumpster reports it's structure.

just to be sure, "Encoding Profile" is what I'm looking for? (Main@L2.1 as an example).

Only you know exactly what you are looking for here, but yes, Main is the profile (indicates the "standdard" features included in the profile) and L2.1 is the Level (indicates specific macroblock, data rate, and resolution) limitations for the profile.

I see. I was hoping a QuickLook plugin could be written that displayed not just the preview of the video (as is default), but also selective properties of the video file, using something like this:

As previously stated, what you see depends on what the calling application was programmed to look for and report. You would need to contact the programmer(s) of the particular application you want to provide the additional information and have them re-write the application. For instance, you might send an enhancement request to Apple requesting they include the particular information you want in the Finder "info" window routines that utilize QuickLook. You could also try to convince someone to write an independent "QuickLook" based application that would provide the information but I honestly fail to see the difference between gathering the desired information directly from QT or indirectly via QuickLook having "native" access to QT structures embedded in the application.