A great mathematics colloquium is one which satisfies many if not all of its goals, and may have additional benefits which make it great.

A list of potential goals/benefits:

• accessibility: many if not all attendees can understand a talk
• entertainment: attendees enjoy or find the presentation enjoyable
• breadth: the collection of topics covers a wide range of interests
• thematic: there is a common thread or other connection which relates the different talks and draws connections
• stimulating: encourages audience members to discuss and extend the subject matter outside the colloquium
• durable: the recording made is archive quality and can be shared with others
• effective: members of the audience can take some of the main ideas and recapitulate them for others who were not at the colloquium, and make the others understand the ideas.

Of the list above, I value being effective more highly than the other items.

Gerhard "What Are Your Colloquium Goals?" Paseman, 2015.06.07