On webpages generated from user generated content, I create slugs for the URLs based on the title (just like stackoverflow does):
If there's more than one slug "day_at_park", I'll append a count:
Does it make sense to include, in this example, names of people involved in the content. So if it was a photo of Susan and Bob, the URL might be:
One benefit is far fewer chances of needing to append a count like the second example above. The URLs also are more descriptive about the core parts of the content.
On the downside, the URLs are clearly longer. And I have to introduce more ugly commas (though better than - or _). If, in the above example, there are many people in a photo... I would have to cut down the slug size by removing people... that seems inappropriate, basically attributing more SEO importance to arbitrary people.