So, the "deluxe edition" contains:
- OST in MP3 and WAV formats - MP3 is understandable, but why WAVs? It's like someone didn't know about loseless compression (FLAC?). Files are inconsistently named, some filenames have extra space, some have missing spaces, I'd expect more from Larian. However, the worst thing is - MP3s are not tagged. No title, no artist, no nothing (well, copyright is there). This is completely unacceptable. Divinity 2's OST was fine, so what happened?
- Artbook - nice, it's cool to see how character's design changed. Nothing to complain here.
- Character sheets - Well, those would make sense if you were selling boxed game, otherwise it's nothing special.
- Adventurer's pouch - completely, absolutely unacceptable. It borders on Pay2Win, even if the items are not that powerful.
- Items from Divinity 2 - Mostly cosmetic, except for Fane's helmet which allows you to shapeshift. Some classes already have that skill, but this item makes it completely useless.
- Dice skin - I rolled mindflayer's head and had to wait for the animation to finish to know what it's supposed to mean. It'd look better if colors were less saturated, more "realistic".
In short, if you want to legally own OST and artbook, get it (and spend some time tagging MP3s).