icons - Consistent UI experience

Shouldn't a user interface be as consistent as possible?

I believe when a user uses an interface, he experiences certain behavior from the interface, and tries to couple those experiences to the rest of the interface as well.

As an example (and the reason why I posted this question) take the accept answer icons on Programmers.SE.

When looking at these icons, I wonder what the checkmark means. I notice the up and downvote icons are either filled in or not. The filled in icon is filled in since I upvoted. I can't couple this behavior to the checkmark, since it doesn't have an outline. What is the current state of the checkmark?

Here I accepted an answer, and notice the checkmark 'lights up'. When comparing to an answer which I didn't accept, I can clearly see the difference that I accepted this answer, and not the other. There is a problem however, when I want to see whether I accepted an answer, and it is the only one there.

So, I believe it would be a big improvement for the accept answer icon to also be outlined. Of course I posted this on Programmers.SE meta as well.

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Obviously I agree with the highest upvoted answer, but I was hoping more on a 'scientific' UX answer, perhaps some terminology, or influential articles discussing this topic.

So to rephrase the question: Shouldn't a user interface be as consistent as possible?

Answer

Human-Computer Interaction (HCI) research often refers to transparency as an indication of the clarity and usability of a user interface. Ironically, it's the lack of noticing the user interface which makes it transparent. When it distracts you, as in this question, it is less transparent.

For my thesis, I am looking into Activity Theory which is a psychological theoretical framework which has been applied in recent HCI research. While reading through the book "Acting with Technology: Activity Theory and Interaction Design" by Victor Kaptelinin and Bonnie A. Nardi I came across an interesting section which reminded me of this question I posed a while ago.

In Activity Theory - as the name suggests - activity is placed central in studies which try to understand the way how people act. In this framework, any task, or activity, can be broken down into actions, which are further subdivided into operations.

"Activity theory argues that actions and activities are usually consciously planned, while operations are performed subconsciously and without deliberation." This isn't a fixed hierarchy. Activity Theory focuses a lot on the developmental aspect of the human mind. The process of an action turning into an operation is called automatization. An example of this is when learning to "touch type". At first finding the correct key is considered to be an action, but afterwards it turns into an operation.

So far the background information. The point is, transparency can be accomplished through skill automatization. Or less theoretically put, skill comes with experience, so transparency isn't a fixed property of a system, but dependant on the user and his experience with the system.

Finally to answer the question "Shouldn't a user interface be as consistent as possible?"

Although transparency can't be "built" into the system, designers can facilitate skill automatization, resulting in transparency. One such way to support the user in turning actions into operations is by using consistent affordances. Inconsistencies prevent or slow down the user from doing this, or even turn existing operations back into conscious actions.