We have noticed that 'Fields' appear on a global basis - that is, across all projects.
The issue we have found is that increasingly, we are creating fields which are per-project. These fields then pollute the field list for all other projects.
At the moment things are not too much of a problem, though we are thinking ahead a little and can see how things will become really messy.
Isaac mentioned that fields are global and there are thoughts on making this list something that each project can cull. That sounds great.
My only thought is that by default, each project should be opt-out - except for the project creating the field.