Some differences in the new printings:
Main Rulebook - There is a new (and good) introductory module included. Minor changes to several classes. Elementalists got a significant boost by being able to channel raw elemental power as a magical attack (increases by level). Assassins get to select their skills from various pools, rather than automatically getting set powers at each level. An optional rules section which outlines minions (4e-esque), critical hits, permanent injuries and so forth. Far better organization of the rules in general.
Bestiary - This is kind of optional, as the new Main Rulebook has the basic stats for all the creatures in the back. However, there is a lot of fluff here... which is kind of nice for the DM.
Sleeping Gods - First module compilation. This hasn't changed much from the original modules (it covers all the modules released in the original books, except for the Elven Crystals and the Power of Darkness).
Elven Crystals - Second module compliation. This has changed a LOT. It may not seem obvious on the surface, but a ton of new background text has been added into the modules, and some encounters have changed significantly. The demon-lord Rimfax is made much more prominent as the "big bad" throughout all the modules. If you have any interest in running Elven Crystals (even if you stick with your "old" books), I'd consider picking this one up.
Power of Darkness (and any subsequent books) haven't been released yet. I'm hoping PoD gets the same update treatment that Elven Crystals received.
