Scott Sehlhorst wrote a really good defense of good product roadmaps in Features do not a Product Roadmap Make:

If your roadmap says “Will include update quantity control in shopping cart” you’re doing it wrong.  Your roadmap should say “Improved shopping experience on mobile” or “Better shopping experience for spearfisher  persona.” […]

When a roadmap is being used to communicate “what” the product will be, it should be in the language of describing which problems will be addressed, for whom, or in what context.  This is the most important type of theme which would be part of a thematic roadmap.  Other themes could be “improve our positioning relative to competitor X” or “fill in a missing component in our portfolio strategy.”

And this, in the context of agile, is a great point as well:

A backlog – a prioritized list of features – is not a roadmap. It is a reflection of a set of design choices which happen to fulfill in product what the roadmap sets out as a manifestation of strategy.

A roadmap tells you both “why” and “what;” a backlog tells you only “what.”

This reminds me of an article I wrote in 2011 called Product roadmaps are safe. Good times.