When given such a task, I always follow these ground rules:
- Remember they came to hear the speaker, not the person introducing the speaker.
- Omit anything that might be remotely embarrassing to the speaker or to anyone in the audience.
- Mention the topic but don't go further into the subject. That is the speaker's realm and you don't want to infringe on any turf. [I once saw an introduction that included an amusing story. Unfortunately, that same story was a key part of the speaker's presentation.]
- Don't set the bar too high in your description of the speaker's knowledge or eloquence. Doing so makes it all the harder to please the audience.
- Keep it short and then fade away. The speaker is the star.