The autofill is utterly useless because it shows pretty much every iteration that anyone has ever used. This means misspellings, multiple different versions of what should be one tag, anything you start to put in brings up way too many options.
The tag suggestions need to be a smaller set so that people end up using the same tag as each other and are actually searchable once again. The large list of tags that we had is a great place to start, especially as we can still ignore the suggestions and type our own things.
Otherwise, these suggestions really don't help solve the discoverability issue at all.