A decent amount of stuff can be fixed by the community Unofficial Skyrim Patch http://skyrim.nexusmods.com/mods/19, including Quest fixes, but theres a just as big amount of stuff that needs fixing from Bethesda because they're inheirant flaws of the code in the executable, like

• NPCs not having enough time to complete their dialogue.
  
• NPCs interrupting themselves because the player bumped into them or got too close.
  
• The issue that causes Khajiit tails to wave even when dead.
  
• Objects making the wrong noises when picked up (provided they have the correct sound setting in the CK).
  
• The way the player clips and sometimes becomes invisible when riding horses in confined spaces.
  
• The way mounting a horse in the water will cause it to move as though you're on land.
  
• Unowned items being marked stolen when your followers pick them up.
  
• Being entirely helpless while swimming.
  
• The way items sometimes just blip out when you're staring right at them.
  
• Mouse lag in the menu to load a game.
  
• That outline around fire effects that showed up after Patch 1.6.

I'd ESPECIALLY like to be able to use my weapons underwater please, and maybe possibly while jumping/falling. It was in both of the previous games and you have put enemies that can kill you in the water, so it would only make sense to be able to defend yourself with spells, daggers, etc.