One of the things that I, personally, really like in iGTD is the 1-5 priority rating for tasks. While I realize this is bordering on heresy to GTD purists, it's absolutely essential to my workflow.
In my system, I have somewhere around 800 tasks that would all fit in the context @Online (I'm a network engineer). Without some kind of priority system, I'm hosed when it comes to figuring out what to do out of that pile.
In Kinkless, I solved this by creating @Online-P1, @Online-P2, etc. to indicate priority. In iGTD, I make heavy use of the priority fields. It's awesome because I can keep all my tasks in the @Online context for easy scanning and review, but still have them ordered.
Even if it's optional, I definitely would love it if OmniFocus had some type of priority rating for tasks.