Things are never simple
You haven't been told about the problems of running multiple quests or why you shouldn't delete a quest. And in a fit of frustration you hit that delete button. Now you find yourself not able to get that master crafted hammer or even worse you can't get into the winch through the quests you were on.
Running multiple quests is no longer the cause of problems. We are even checking if it would be ok to have more quests open with the same NPC. The problem here seems to be the delete button. Would be easiest to remove that

Here is my solution. #1, Make it so that a quest that is deleted is automatically reset. After all you didn't finish it. thus you didn't get the reward. This means you can start over and try again correcting the mistakes or even verifying that there is a problem with the quest. This won't cause you to be able to redo quests that you finish. Only those that you didn't complete. Or if you don't want to have a delete, make a reset button.
As Toox already explained, there are quests that give a reward before you finished it, which would make resetting an easy exploit. Another thing is that many quests now have different routes. Being able to reset would mean that you could start again after finding that you took the less profitable path.
This would remove a lot of frustration that new players find in the questing system and now your GM's wouldn't have to deal with reset requests. Note I am only saying reset the current quest your on. Not reset a multi quest sequence all the way to the start.
I think 99.9% of quests are multi quest sequences. It is the way they are defined. Many quests have others as prerequisite, which may be what you mean by multi quest sequence. These are never influenced by a reset anyway.
The best way to solve the frustration (for players and GMs) is to fix the problems. Most of the problems with quests at the moment are with errors in the scripts, or answers that are very difficult to guess. Best way to get those fixed is to describe the problem in the bug tracker and set the category to settings. The very active settings team can than fix the problem and prevent the need of resetting for others. Try to describe the problem without giving away spoilers, or just open a report and ask it to be made private, after which you can give as detailed information as you want.