How can I remove a tag from a resource/group/my profile/etc?
When editing content, your profile, groups, etc. you can remove tags by simply deleting them from the list presented in the "Tags" form field.
Is there a limit on the number of tags I can add to an item?
No. You should be able to add as many tags as you want to taggable content/profiles.
Can I tell who's tagged another member's content?
No. Who tagged an item and what tags they added is currently not available to users.