Small changes in functionality can make big impacts on usefulness, and therefore the value of tools and applications. This has always been true, but is more obvious on mobile devices where limited user input, network access, and screen size magnify the value of a good user experience (UX).
One example of this phenomenon is Wikipedia. On the iPhone, I can look up data on Wikipedia using Safari, or using one of the Wikipedia iPhone applications. At first glance, an iPhone Wikipedia application is absurd. After all, it simply uses the built in Safari to render public web pages to the user. The exact same pages can be viewed using the built in Safari browser, and you can add a bookmark directly to the Wikipedia page to your iPhone home screen. But one difference makes all the difference. That one difference is auto-completion. On the mobile site, Wikipedia does not provide for search auto-completion. You must type in a full search and execute it. If you searched correctly, you can then select the result and view the page. Using the application, you simply start typing, and then select the correct match of your partial search to view the page.
This probably saves less then 30 seconds per usage. But looking up an answer in Wikipedia should be a sub-minute activity, making it a significant difference. And that difference makes me more likely to use the application, increasing its value.
A second example is Instapaper. Instapaper is essentially a bookmarking service. Through various means, you can mark a website for reading later. Then, using various client interfaces (Web, iPhone, etc.) you can read that article later.
When I first read about it, I didn't get it at all. What's the point? There are several existing ways to do the same thing. You can use Delicious, Bookmark items in your browser, email links to yourself, etc. It doesn't really allow you to do anything you couldn't do already.
After a few weeks of usage, I found that I am reading many more long articles with better reading comprehension. Instead of hurrying through an article when I wasn't dedicating my full attention, or just skipping it because I wasn't 'that interested', I now Instapaper it, and read it when I'm ready.
The functionality provided by Instapaper and the iPhone Wikipedia application are both trivial enhancements over existing options. But they provide just enough grease to make a task that was possible before a little easier. That can be a huge value.
When looking for that next great idea, remember, you don't need to invent an amazing new product category, you just need to make one thing easier to use.