I noticed nobody mentioned Extensis Portfolio. It's not all that cheap, but it has *all* the features you want.
1. Fast and reposnsive - Initial synchronization isn't fast at all, but searches are lightning fast.
2. Ability to select a folder in the tree view and see all pics in that and its subfolders - Integrated with #5.
3. Tagging support for XMP and IPTC - Covered. Here's some info on XMP: http://dl.extensis.c...lio_XMP_Workflow.pdf
4. A proper way to organize/filter views based on tags - They are named keywords on Portfolio, but work the same way.
5. File monitoring for new folders/pics - Of course.
6. A nice UI with support for multiple zoom levels - You can pretty much customize the way files are displayed to your heart's desire (using different "views").