Just a couple of thought because, as Lucas offered, the subject of tags (and task priorities) has been beaten to death. Tagging offers some interesting options to add rich meta-data to your tasks, and I can understand how it is/can be beneficial. Things has tags, OmniFocus currently does not. The OmniGroup has indicated that they will be looking at adding the ability to input user meta-data in OmniFocus 2.0, but for now it is what it is.
Bottom line is that Things also has a Today view that is nice, while by default OmniFocus does not (a perspective can get similar functionality). Many people believe that Things has a more attractive UI, but as the OmniFocus UI has themes, many people prefer to customize the UI of OmniFocus. Many people believe that Things has an advantage due to its simplicity, while OmniFocus can be just about as simple or as in-depth as the user wants to make it. Things Touch does have the ability to email tasks while OmniFocus iPhone does not.
On the flip side, OmniFocus has parallel and sequential tasks, sub-projects, cloud syncing, project review functionality, custom perspectives, custom toolbar with built-in commands and scripts, scheduled tasks on the Mac and the iPhone with start times, due dates with due times, alarms through Growl/Prowl, and iCal, bookmarklet to add tasks from Safari, the iPhone has Locations Services customized to contexts, imbedded browser, photo support, voice note support, repeat task support, and a completely customizable interface through the use of synced perspectives and icons while Things Mac and Things Touch... do not.
In the end, it all comes down to what features are most important to the user. If tags are essential to your workflow, then I doubt any of the solutions that the OmniFocus users have will make a difference in your decision. However, if the areas where OmniFocus really excels are important, then there are other ways to get much, if not all, of the functionality of tagging.
Using the Bob/hardware example, let me share a few ways that I might handle this:
1) Due Date: If Bob is going to fire me if I forget to buy xyz at the hardware store, then I'm going to assign a due date to complete the task. If I am running errands and I see the task a week early when I check my @errands context, then I go ahead and complete the task. If I have not run errands in a while, the due date arrives, and I have not yet bought the item, then I'm on notice from the due date that I'll be making a special trip to the hardware store.
2) Daily Review: I examine my projects to see where I am with my active projects and what I need to focus on today. If I were to see that the hardware item must be purchased in the next 4 days and I'll only have the time to run the errand today, then I'm going to block the time on my calendar to do it today.
Personally I find that the daily review, judicious use of start and due dates, and flags in addition to contexts gives me everything I need to keep tasks from falling through the cracks.