As a partial answer: Allow for a 10 minute break in the middle of a long presentation. It is rare for someone to follow for two hours straight, and many topics can not be done thoroughly in less time. Also, the break time can be just as productive as the presentation.
Towards the parenthetical remarks about success: It is good to have a clearly defined goal. If everyone is on board with it, then set policy that helps everyone acheive the goal. In your example, I would limit/prohibit any side discussion that is not helping to educate in the topic being presented. This implies that questions about applications to other fields, or alternative definitions or problems that take one outside the scope of the presentation ( even if it is covered in some other presentation ), and their responses, should be brief or be cut short. (I would not apply this prohibition if the goal were to stimulate new research or creative problem solving.)
One opportunity I missed out on was when I was an undergraduate sitting in a small graduate PDE class looking at a paper by Leray on Navier-Stokes. The students took turns translating and presenting sections of the paper. If I had been smarter, I would have met outside the seminar with the graduate students to go over sections of the paper and help improve my and their presentations. If I had done that, I would likely remember the mathematical impact of that paper, rather than forget most of the technical details needed to push through the proofs. Perhaps something similar would be effective for your "Classics" seminar: team presentations.
Gerhard "Ask Me About System Design" Paseman, 2010.02.20