Because release 0.10.0 makes use of headers-first synchronization and parallel block download (see further), the block files and databases are not backwards-compatible with older versions of Bitcoin Core or other software:
Blocks will be stored on disk out of order (in the order they are received, really), which makes it incompatible with some tools or other programs. Reindexing using earlier versions will also not work anymore as a result of this.
The block index database will now hold headers for which no block is stored on disk, which earlier versions won't support.
How exactly does the new blockchain format effect blockchain parsing projects (of
blkxxxx.dat files) which likely rely on the old format? Is there a work-around? Are there any other specifics worth mentioning?
Examples of specific projects: