I really like using a sketchy visual theme for wireframes. They purposely convey a feeling of roughness. Incompleteness is actually a good concept to leverage when you still have mockups to build later.
Microsoft Visio may not be the best tool for wireframing, but it suffices. To make it even better, Henrik Olsen and Niklas Wolkert built some fantastic templates and sketchy "shapes" for Visio. Jonathon Abbot created an additional template that improves upon this work. This template combined with the Hand of Sean font (or some of these other web designer fonts), can create great results. I have posted my own Visio file as well.
UPDATE: Per Jonathon's comment, I am now giving proper credit to Olsen & Wolkert.