We talked before about porticoes and towers but Bologna is so much else about architecture.
San Pietro Cathedral in Rome and San Marco in Venice are really astonishing churches but in the armony of Piazza Maggiore you'll remain marveled by the beauty of San Petronio's front (photo above). Unfortunately it's covered for restructure since 2011 but that gives you the chance to rise to the terrace and have a wonderful view of the square and the red roofs of Bologna.
If we talk about churches you can't miss a visit to Santo Stefano Basilica (the so called "7 churches" complex is a unique example a romanic architecture) and the rise the 469 steps to the top of San Luca, or a walk under the arcades of Santa Maria dei Servi or the profane Chiesa del Baraccano (photos below).
But architecture in Bologna is still a lot more. I can talk for hours about the front of Palazzo Pepoli or Palazzo della Mercanzia but my suggestion is just to walk and relish of your findings. Turn up your eyes to discover amazing windows or unexpected facades, get lost in the tiny streets of the ancient Jewish ghetto or peek inside Collegio di Spagna (photo below) to admire its wonderful courtyard.
Bologna is a city to discover with a slow tour, walking around and taking your time to rest and have a coffee. After 28 years I still get surprised finding out all the beauty of my town.
[photos courtesy of Andrea Forni http://www.flickr.com/photos/forni/]