Heh, we used to have threads like this all the time. It's been a while, so it's refreshing to start up some new discussions along these lines.
If we go with a very broad list of virtues (as opposed to one of the "codified" ones out there), I'd say my biggest weakness is Perseverance. I have a bad habit of getting 90% of the way through something (hobby projects, home improvement, whatever), and then grinding to a halt. It's easy to get burnt out on something, but often it also comes down to other things, perhaps of much higher importance, coming along and demanding my time. Once the momentum is lost, it is difficult to regain.