Make Enter behave like a normal form. Currently it just auto-completes tags (like Tab). It should add the selected tag to the list, clear the box and let you continue adding more tags.
Here's what you need to do to add two tags to a filter list now:
tag1 -> Tab -> tag2 -> Tab -> Tab -> Enter/Space
Notice the two superfluous Tabs.
Here's what I'm proposing:
tag1 -> Enter -> tag2 -> Enter
Not only is this faster, but for the 90% case of just adding a single tag to the list it's 2x as fast!
tag1 -> Enter
tag1 -> Tab -> Tab -> Enter