None. If you decide to do the Southern Shelf quests later, you just have to go to Clappy or Hammerlock in Sanctuary to turn them in.Are there any side quests that are actually missable as the story progresses? (like due to the NPC moving location or whatever, ie. Hammerlock/Claptrap in Southern Shelf with the quest where you turn in Bullymong Fur to one of them)
If there are, I might as well do them in PT2 rather than skipping
There is a case where a quest-giver dies later on in the story, but it's inconsequential since the quest was posted on the bulletin board in Sanctuary and gets turned in there regardless of whether the original quest-giver is alive or not.