Why isn't iTunes Radio presenting the options affecting station variety?

It only works if you add your own custom stations. When you are listening to a song that you like, press on start station from artist or song. Then that new station will have the options on: play more like this, or never play this song. :)

According the Apple support doc for iTunes Radio:

There may be some DJ-curated stations that you may not be able to modify. You can't like or play more tracks like this while listening to some DJ-curated stations. You can however, start new stations based on the current track or artist of a featured station or add tracks to your iTunes Wishlist from the Star (✭) menu.

As for the slider you can only use it on non-DJ-curated stations. Create a station from an artist or song then you will see the slider.