The immersive elements, what is your opinion?

So, about two weeks ago I made a mistake and now I am heavily engrossed in researching all about immersion in games. Some very interesting opinions sparks from this topic and many do go with some extreme answers on how "real" immersion should be done. 
So, in a nutshell, I am asking this lovely community some simple questions. What is immersion for you? What is the driving force, the mechanics and aesthetics/concepts, that make a game immersive? You may or may not really think abut immersion as something of value in games, but that is an opinion/argument I also want to hear. Why is immersion overrated/underrated? 
As a last question that may be the most important one: What breaks the immersion for you? What is usually the irritating thing that just destroys your suspension of disbelief? 
All this has probably been debated endlessly on all kinds of boards, forums and threads. But see this as my personal little therapy session. 


Well, thank you for being part of this.

Maybe not even blog-worthy-esque but I feel the rush, no the duty, to write it just to say: Thank you. Been a horrible person (non-member) for too long now. This site is one of the best with its content and overall style and humor, a risky claim to throw out on the vast internet. 
Well, anyway, I am now tangled deep into this mess and want you all to know that I will drag you all down with me. 
Babel Tower.