The typical user story starts something like
As a <persona>, I should be able to <task to complete>
That’s a decent format, or at least it’s a minimum bar. Without identifying the kind of user and the task to complete it’s really hard to compare tasks to identify which has the highest value. However, it is by no means a guarantee.
There are 2 really common reasons that this template doesn’t guarantee that you end up with a quality user story that you can implement. The first is that there’s no mechanism to ensure that the task the user is trying to complete is actually a task that adds business value and not just an activity along the way to completing a task that adds value.
The canonical example of this is the login user story. You know, the one that reads something like
As a return website customer I should be able to login to the website.
That story hits all of the requirements of a user story. The customer is clear. The task is clear. Acceptance criteria, while not explicitly listed, is clear. Even with all that, it’s not a good user story. It’s not a good user story because the task, logging in, doesn’t add any value. It’s an activity that needs to be done, so by all means, do it. But if all you did was add the ability to log in to the site then you wouldn’t be adding value. You’d also be making your customer’s life worse. They would need to jump through some hoops, but things wouldn’t be any better.
A better user story, with a task that adds value, would be something like
As a return website customer I should be able to order the contents of my shopping cart using my securely stored personal information.
Do you need to be able to log in? Maybe. You need to access securely stored information. Logging in is one way to do that. But there are other mechanisms for authentication. The user story is not the place to specify the mechanism. The team should figure out the mechanism by looking at the situation. And once they have the mechanism determined/implemented, they can then use it to ensure that secure personal information is used. The customer doesn’t need to enter information every time and you don’t need to send the info over the wire making things even more secure. And that adds user value. When the story is done the user is measurably happier.
The other common reason, and the one I see more often, goes the other way. The story is too broad.
As a return website customer I should be able to buy something
That is certainly something a customer wants to do, and doing it adds value, but where does the story start and end? That story covers just about every e-commerce site ever made. To really be useful and completable in a reasonable amount of time it needs to be split up into many more much smaller stories or scoped down to what the story really means. That story is probably a conflation of at least stories for browsing, adding items to a cart, setting delivery address, and making payment. I needs to be split up into those stories. The challenge there is to not split the story up so far that it turns into a task list (see above).
Right-sizing stories is the key to making useful user stories. Making them big enough to actually add value, but not so big that they can never be completed. Tim Ottinger has a good list of resources for splitting stories.