Water
Rain/Thunderstorms
Dynamic lighting
Day and night cycles
Lens flare
Glossy and shiny surfaces
Particle effects
Beautiful non-low-res skyboxes
Everything listed here.
Snow and ice.
Shiny caves. Mushrooms are nice, but crystals or lights work too.
On that note, mushroom areas in general.
Also, 2D spritework. I'm not just talking about the top of the line, gorgeous Vanillaware-esque sprites, but anything that manages to look nice. This should actually be at the top of my list. I could show any number of images, but I will say that 2D fighting games usually have fantastic sprite animations.
But even stuff like this:
Maybe because I grew up with games like Super Mario RPG and Star Ocean 2. I know I'm not the only one, though.