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Things to keep
Simplicity of Class Design
Runs at high settings on mediocre computers
Good layout of noob areas and racial cities
Simple "here and now" quests (i.e., a farmer says, "pigs are eating my crops, go kill 10 pigs")
Faction (but reduce number of factions to workable level. There should not be 40+ factions; maybe a dozen would be about right).
Things to ditch
Sci-fi meshed with fantasy (Gnomes making a telescope might be reasonable, but motorcycles are a bit much)
Lame humor referencing the modern world (breaks immersion)
Dalaran (or any "Plane of Knowledge" type city)
Insta-porting (except certain casters, where it makes sense. Travel and dangerous cross-continent journeys are part of the adventure and give a sense of geography to the world. "Where is Conn?" "Don't know, last I heard he ventured into the Southern Deserts." "Okay, we'll have to do this without him, or venture out to join him." "Sure, let's load up on arrows and rations and see who else wants to go?")
Phasing (isolates players)
Instancing (takes "community" out of dungeons. "Community" in dungeons is fun. It is cool to run into another party in a dungeon, or to venture safely inside while following some other party's corpse kills.)
Storyline quests (especially ones involving phasing. These mean nothing, take time, and suck in general).
Cross-server grouping (no reason to socialize with someone you'll never meet again. Sort of like making friends at a bustop or airport. Why?)
Mobs that look incredibly tough but drop in two swipes, and level 70 "deer" (I once saw a huge level 15 stegosaurus standing next to a level 16 giraffe. C'mon.)