For Withermoore, you need to examine the device that opens the barrier with a character that has no Source after you freed Withermoore. If you have Source, it will only give you the option to open it yourself and if you do, the quest will not close. You miss a scene and some exp.
Call to arms does (not) finish correctly depending on your last conversation with the leader of the Seekers. I have not tested enough to see what exactly makes it complete but you don't miss out on anything if you have seen the Seekers run to the coast before the fight at the harbour.
To "finish" the arena quest you have to fight one of your party members which is only possible if you play co-op.
Killing all shriekers with purging wands prevents Slane from helping you out. You miss a small scene and a buff.
The main problem is the wording of the "unfinished" quests after you leave the island.
If you did not collect your last reward from Withermoore or Slane it still says something like "you left the Island without helping <person>". While the incomplete quest is a hint at the things you missed out on (which is good), you still did help them - they just didn't return the favor because you didn't need the help.
