What makes a Shimano STI shifter better than another STI shifter?

I am curious as to know what a "better" Shimano STI shifter would do for me
over the cheaper ones.
As I see it at present, we have the Shimano Tiagra, 105's, Ultegra, and
DuraAce STI shifters.
For example, What would make the DuraAce shifter better than a Tiagra
shifter.
The price difference is pretty big, so what would a DuraAce shifter offer to
make it so much better than a cheaper Tiagra shifter?
Does it use special precision ball bearings versus bushings or something?
Maybe the indexing mechanisim is higher quality or something.
Can you get repair parts for the "good" STI shifters versus the cheaper
ones, are they rebuildable or something?
I am trying to find out what justifies the huge price difference.
Now granted other components can make a big difference, I did notice that a
really nice rear Shimano cassette and a expensive Wipperman chain versus
stock OEM stuff can make a bike shift so much nicer. But I was trying to
determine what going to a "good" STI shifter might bring to the picture.
Obviously if you hop onto a bike at the LBS with all DuraAce components on
it, it is going to shift really well, but it has a good chain, rear cassette
and stuff already, so my\aybe that will mask out what a "good" shifter might
be doing or not doing.
Thanks for reading.