Are those "hooks for scripting library for spells" really something like those weird classes of scripts for spells in tc2?
I don't know if this can truly be better than handling spells in core, grouped by their effects... It would just spread the code and make things less clear.
About the list...
Pets rewriting... The biggest problems are stats and multipets. Stats bug is the easiest to fix Scaling auras are not used (at least not all of them). We need formulae and list of scaling auras and use them, thats all.
Aura stacking is not a big issue, needs only checking of groups of auras by auratypes and like 1 - 2 additional functions for handling that.
But rules of dividing the aura stacking groups is the real big issue for mangos, because "exceptions code" aka HACKZ is not approvable and some spell attributes must be found in dbc or something as stupid as this. ;P Again, playing a researcher comes into the topic...
For me a true milestone was pathfinding/mmaps and a big applaud for that to all the authors and contributors.
Next true milestones would be: general support for checking for GameObjects in Line of Sight for spells and yes, VEHICLES!
Dungeon Finder or Wintergrasp more like features.
Oh and handling movement/animations of creatures like on retail servers would also be a really big step forward with all recent movement changes.