The new water elemental creature is for hire. The barriers and tents are interesting. As a designer you now have a way to force a player to visit one place before being able to pass through another place. Coupled with the use of portals you can make some very mazy levels, and some of the standalone scenarios illustrate that well. There's a good variety in these, with varying winning conditions, showing a good variety in terrain and difficulty.
Ghosts are just way too powerful.
That's very powerful. The Sphere of Negation is a great item for a barbarian to get. Given the number of new artifacts, it's a shame more aren't more creative. The editor and spellbook. The new expansion game editor allows you to use all the new features, but obviously expansion maps using new features will require the expansion pack to run.