In the last year, I've been making more of an effort to speak at user groups both inside my company and in the community at large. I've found I learn very quickly by trying to teach others. So I pose the question to the community, what is more valuable to you?
It depends on your audience.
A meeting for a group or programmers in your company who already are familiar with the topic will prefer you go more deeply.
A local users group meeting will have a wider mix of knowledge levels and interest. You will have to broaden the scoop of your talk.
A meeting of a local business leaders will not want to know how ti works, but what it can do for their companies.
Match you topics and depth and breadth of coverage to your audience. Watch them during your talk to get feedback. Adjust your speech accordingly.
Like anything else it takes practice, keep at it.
For "us" (meaning a technical audience):
You could pick a subject you like, and challenge it (checking how to does actually work behind the scene, looking for not well-known details, discovering some quirks).
Basically, your topic should be sub-titled:
"Everything about 'foobar' you think you knew, but actually did not fully know".
Whatever topic you end up choosing, it should be one that can be "run" live (for instance, a language where you type snipets of code live during your presentation)
What is nice with this kind of presentation is the way you can easily entice your audience to challenge you, asking questions, and demanding live illustrations of the topics you mention.
That way, you can have a true interactive session (as opposed as a professorial one).
Rehearsal is important of course, but a little less than in a "formal presentation": Your ability to think on your feet will be tested ;)
As Jim said, this depends on your audience and the above advice might not be a good one for other crowds.
Open Space Technology can be really good and useful if there is a large enough developer community in your area. Since these are self-run to some extent, the community gets to decide what to discuss and how in a way that I really really liked.
Next on my list would be taking some tool, which can be either a technology or a methodology, and demystifying the power it holds such as showing Dependency Injection or secrets of Resharper or refactoring.
Third would be doing some analysis of technologies out there, showing their strengths and weaknesses within a discussion though this requires having some ground rules so that it doesn't turn into a flame war or fan boys all patting each other's backs.
Lastly, don't forget that you could break something into a few pieces and remember to ask your audience if they would like to see another part in this series or start something else once in a while. Another user group question like this was asked that you should also look at those answers.