For the art:
Pull out "Worlds and Monsters" (the 4e preview book). Look at the landscapes and scenes in there. Then make that your goal for 5e art: to present landscapes and scenes that we can stick adventures in!
I don't need to see any more pictures of a guy waving a sword around, or a half-naked babe casting a spell (though the latter never goes amiss!

), or an orc being macho. I want vistas, scenes, action - and if the occasional adventurer happens to be doing something in the distance, that's fine too. But people (and monsters, and animals, etc.) should be secondary if present at all.
Also, EVERY adventure should come with a picture iconic to that adventure; be it a scene from the dungeon*, the landscape in which the dungeon is set, the nearby village, whatever. The idea here is that picture can go on the outside of the DM screen as a reminder to the players where they are and what they are doing and-or up against.
* - but please avoid giving away any information about said dungeon in the picture!
Lanefan
[edit to add a second to [MENTION=34865]thedungeondelver[/MENTION] regarding Warhammer RPG art - they get it]