"I am glad that you are enthusiastic and I don't doubt your experience. But I am going to push back on you a little."
No problem :D
"ClearCase branching is a lot more powerful and functional than Subversion. Variant management with copy branches in general is different than delta branches (the latter make it much easier to know what files have changed)."
yes, you're right, however in many cases dev team doesn;t need so sophisticated branching strategy with huge version tree and dozens of branch.
Each branch costs so I rather prefer to have them as little as possible.
You can easy check what files changed in SVN too using svn log which is much faster than cleartool find command.
"You also need to deal with the limitations and issues with merging in Subversion that are much easier and clearer in ClearCase."
Less branches you have less merges you need to perform :D I agree that merging (especially tracking merges) in SVN is still a hassle but keeping reasonable numbers of branches makes developer's life easier. So if you want to keep all your changes on separately branches do not choose SVN since it's not so convenient to track merges as in CC.
"An experienced ClearCase user who understands how to use the product is going to find merging in Subversion to be a bit different. Did you address those challenges and - if so - how?"
Yep, I did... First what I showed to them was that they don't need to create branch for each bug, so they don't need to merge so often. Then I explained to them that from version 1.5 SVN has tracking merges feature (each merge is saved into SVN property- svn:mergeinfo). I admit it's not as convenient as ClearCase graphical version tree but I bet after some time of working with SVN you forget about version tree.
In general I'm CM for 6 teams, 3 of them using ClearCase (under unix and windows by means of Remote Client) and 3 are on SVN. Personally I love CC under unix and hate it under Windows (cause of CC Remote Client). I plan to move to SVN world another windows project so it will be 4:3 to SVN ;)
I'm not saying that SVN is the best SCM tool in the world. It depends on many things... But I don;t want to argue here which tool is better. Question was about migration from CC to SVN so it seems that it's decided here which tool is more convenient for your team, right? :D