In prior versions of CRM you had to decide at the time you created an entity if you wanted things like Notes, Activities etc. enabled. Now in CRM 2011 you can defer that decision and make it anytime in the future. When you had to make a final decision at time of entity creation it used to be always a good idea to turn them on if you “Think” you might ever need them. Now, I would say the opposite, don’t turn them on till you need them.
Keep in mind that the + after the item indicates that once you turn it on it can’t be turned off.
Sometimes the toughest part of learning a new version is forgetting old version habits!