I have an overall planned story for my character, part of which runs through the dark brotherhood. One of the first sets of contracts you do for them include killing one person that I know starts a small quest (very small in this case). I am wondering if I should be aware of any significant events that these contracts might remove from the game. Hover the spoiler to see the small one I mention above.
For what its worth, Google led me to a few articles on reasons not to join the Dark Brotherhood that all ended up being speculation, and a post on the gamefaq forums that also did not provide useful information.
One of the first people you kill is Narfi whom you can search for his sister if you are not out to get him. This is a very short/minor quest (with not to bad rewards actually) however.