idea for improvement
 

sorry for my broken english...

I work für 1 month with omnifocus and I think it's great. But: to give a date for an activity oder a projekt could be easier in my opinion.

It is so easy to put the activities in the postbox and then attach to a project in the list on left. After that attach all the new activities to a context.
I wish the third step could be - in the same way - attach the non dated activities to a choice of dates on the bar left. Maybe this day, next day, next week, this month and so on. Not every acitivity must have an exactly date.

Or: is this possible but I didn't find it yet.

You can specify dates by entering things like "+1w" for one week from now, "+3d" for 3 days from now, "Tuesday +3w" for 3 weeks from Tuesday, etc. Not quite what you are requesting, but perhaps sufficient? OmniFocus doesn't have a notion of a "fuzzy" date, so an action or a project either has an exact date/time, or none at all, for start/due/completed. The shorthand I mentioned above at least saves you from having to figure out the what the date of "three weeks from Tuesday" is.

See "setting start dates" in the Help command for more examples.