Mr. OmniFocus aka David Sparks started a new series on his blog about the hurdles one might face when dealing with task management systems. I recently switched from Things to OmniFocus and faced some problems as well. After years with the German task management application I had acquired a workflow which did not translate well to OmniFocus.
In Things, everything's basically always available and you schedule tasks for certain days or hand pick the ones you want to work on on a certain day. In OmniFocus, it makes more sense to defer everything until you realistically think that you'll get around to working on it. This way, tasks will only be visible to you when you really need to take action. This of course is a setting and can be adjusted as needed.
This became most clear to me when I started looking for an equivalent of Things's Today View. There is no such thing. With the pro version of OmniFocus it's fairly easy to create a perspective that will show you all available items. The Forecast (available in the regular version as well) will show everything that becomes available that day. If you forget to tick something off that day it won't be there the next. So this is where the 'available list' comes in handy.
David recently did a course for Lynda.com on OmniFocus in which he explains how he set up his today list among others.