Just to note, Pydev 1.5.3 was a 1 bug fix release, fixing an annoying bug (an error would be print to the console when running on python -- although everything would still work as expected).
As for 1.5.2, the changes were focused a lot on memory and performance improvements (special thanks to the Eclipse Memory Analyzer, it really helped a lot).
Basically, Pydev had some memory peaks when analyzing modules, because it expected Eclipse to properly manage Jobs created (which are basically threads managed by Eclipse), but it turns out things were not so straightforward, so, say that you changed 500 files (in a source-format operation), it would happily go on and try to create 500 threads in the analysis (that's now fixed).
Other memory savers were targeted at handling Strings (which are 'sort of' internalized now when parsing the code, but without using PermGen space). And not keeping references to Tokens from the parsing alive (because any reference to any Token would keep all alive).
On the performance side, some changes were done to the grammar to make it a bit faster (that area had already been profiled a lot, so, there are not many performance improvement opportunities there).
Aside from that, the compare editor now has proper syntax highlighting, indentation handling and code completion from the Pydev editor. And as usual, some other minor niceties and bug fixes are available.