Also look at the DMG page 44, in the Hindering Terrain section: "Targets forced into hindering terrain receive a saving throw immediately before entering the unsafe square they are forced into. Success leaves the target prone at the edge of the square before entering the unsafe square."
later on the page, in the clarifying text, it says, "Targets of forced movement in hindering terrain (pits, precipices, fire) can avoid plunging into a pit or over the edge of a cliff or being pushed into a raging fire."
Looking at the definition for Hindering Terrain (page 61) gives us:
"Hindering Terrain prevents movement (or severely punishes it) or damages creatures that enter it, but allows line of sight."
So, yes you get a save for a Pool of acid, a River of lava, or Just a regular river.
Technically, a wall of fire and cloud of daggers are not terrain at all, so no.
When I DM, I ignore the 'But allows line of sight' phrase, because many things that should be considered hindering terrain (like Walls of fire) are not technically hindering terrain only because they do not allow line of sight. Actually, I consider that phrase to be 'but allows line of effect'. So I allow saves for any thing that could damage a creature or hinder thier movement 'severly' (ie. more than difficult terrain)...including spell effects that are not technically terrain (but IMO, should be treated as terrain)