Use Actions and Filters
If you think people would like to add or alter some data: provide apply_filters() before returning.
P.S. One thing I find a bit disappointing and that your question addresses is the percentage of plugins that are designed only for end-users, i.e. that have no hooks of their own. Imagine if WordPress were designed like most plugins? It would be inflexible and a very niche solution.
Maybe things would be different if WordPress were to have the ability to auto-install plugins on which other plugins depended? As it is I typically have to write a lot of the functionality I need from scratch because clients want things a certain way and the available plugins, while 90% there, don’t allow me the flexibility to update the remaining 10%.
I really do wish those leading the WordPress community would identify a way to ensure that plugins are rewarded for following best practices (such as adding in hooks for other developers) much like good answers are rewarded on a StackExchange site.
Let’s take an example from another question:
Example: I want to do something in my plugin when someone retweets an article. If there was a custom hook in whatever the popular retweet plugin is that I could hook in to and fire off of, that would be great. There isn’t, so I can modify their plugin to include it, but that only works for my copy, and I don’t want to try to redistribute that.