After lots of articles explaining why I hate Perforce I thought it only fair to write a few explaining some of the things I like about it. I'm sure that other version control systems do a better job than Perforce does with some of these things but in my opinion Perforce does them better than CVS and current stable versions of Subversion at least.
Perforce keeps track of what previous changes have been merged (or integrated) into a working tree and commits this information along with the files when the files are submitted. This means that it is often trivial to merge changes in from a branch, do a quick build to check that everything is fine and then check them in.
Although it doesn't excuse the submit command not taking multiple filename arguments I think that I mostly like the idea of being able to group my changed files into change lists. The change list can be created and the diff checked over before finally checking it in. There's a certain risk of failing to check important files in if they also happen to be in a different long-lived changelist with this tactic though. It would be ideal if somehow change lists could be at less than file granularity but I'm not sure how this could be implemented without offering a list of patch hunks.
P4P is essential when working remotely with a large depot. It intelligently caches file revisions that pass through it so that future requests for those files can just be retrieved from the cache greatly increasing performance and reducing network traffic. It's not perfect in that if you submit a file through the proxy it doesn't appear to cache the contents immediately thus forcing a further download of the time. Nevertheless if you have many clients or multiple users in the same location then a proxy is worth the tiny amount of effort it takes to set it up.
Update 2008/11/17: Subversion 1.5 now supports (to some degree) all of these features.