2. Non-deferred unique checks
Merging two sets of data and renumbering pkeys is more fiddly than it
needs to be. Doesn't happen often, but it's one of the few things we
don't do "properly".
3. Date handling
Sometimes I've got data with invalid dates and it would be great if it
could replace all the bad ones with, say "0000-00-00".
You could throw in non-updateable views, exact-match tsearch queries,
per-statement triggers not having the old/new rowsets but that's more
things PG doesn't do rather than things it does.