A future version of OF will include custom meta-data fields that will let you do exactly that. In the interim, some people use the estimated time column as a meta-data column.
I would love to be able to make use of a priority field, myself, for the purpose of filtering the dozens of (mostly single-action) tasks I have in several of my contexts. Designating certain tasks as "low energy" would also be useful, as David Allen suggests (doing a few "low energy" tasks often gives me the energy to tackle bigger ones).
While I agree that priority is often a relative thing, I still find it useful to demarcate certain (usually single-action) tasks as high or medium priority (or preferably more arbitrary values like numbers, etc). I'm using flagging to designate "high priority" items now, but it's just not granular enough - I'm still seeing *way* too many items.
As for that priority changing over time, I don't think the software needs to worry about that, because as FranklinCovey teaches, there's a difference between Priority (how important it is to do something) and Urgency (how quickly you need to do something). Urgency is already taken care of by the "due date" column, which you can use for sorting/filtering, and OF does nice color highlighting for "due soon/overdue" items, so priority never needs to change based on urgency. Now, having your priorities change based on the priorities of other/new tasks... well, I suppose that can't be avoided. Projects do allow their tasks to be ordered, which is usually how I prioritize things, but it would be nice to have "buckets" to reduce the work involved in having/maintaining a specific order.
Having assignable "tags" would be another good solution to both the "low energy" and "priority" attributes, although my hunch is that custom meta-data fields might prove more useful & easier to maintain. I certainly *don't* want to litter the interface with tags as Things has done, but being able to add a couple extra "custom" columns & decide which ones to show/hide -- that would be great.