Graphics technology has come a long way since the medium's debut, of course, but seeing old games next to their modern counterparts really highlights the improvements. Handheld platforms in particular have come a long way, and it's fun to compare the old and the new.
A post on Reddit gifted us with the series of comparisons, and I've drawn a couple of additions from the comments as well. The series represented include Pokemon, Super Mario Bros., The Legend of Zelda, Call of Duty and more.
The individual differences are too obvious to even need to point them out--the games are completely improved in all aspects of textures, lighting, resolution and effects.
You can debate the choices of comparison (another top-down Zelda was used rather than a more visually impressive 3D game, for example), but the point gets across regardless.
Games like Call of Duty are of course striving more for realism than, say, Super Mario Bros., but even the increase in highly-stylized cartoonish graphics is admirable (though the creator didn't choose the most recent Mario game, either). When you really look at the Call of Duty textures in 2003's game compared to how good Call of Duty: Ghosts, it becomes clear how far we've come.
Pokemon is definitely among the most easily recognizable improvements at a glance, though. Many will have fond memories of Pokemon Red and Blue, but the graphics have advanced considerably. To be fair, the latest improvements in Pokemon X and Y were still pretty big jumps from the last few games in the series--they finally made real, 3D battles and environments.
Enjoy the screenshots, and check the thread for some commentary and other entries. One day, we'll probably look back on the current graphical achievements with a similar reaction, which is something to think about.