What improvements did you hear in going from entry level to high end Audio?

I heard more detail. Better transparency and detail 
I am glad that you enjoy your system Calvin.  That’s all that matters.  
I am in a good place as well.  I have had more expensive demo stuff in my system, and it didn’t work.  Not in it for “my stuff costs more than yours” factor.
And I truly believe that I am at the asymptotic part of the curve, where shelling out a lot of extra cash will only marginally improve things.
  For a long time my base system was my College dorm system—Advent 3 bookshelf speakers, an entry level Pioneer receiver, entry level turntables, cassette recorders, and then a Sony 14 bit CDP.  When I finally started upgrading, each successive change was huge.  Now, not so much
Mahler I agree.  Now I am where I am if I spent more the system would only probably get small fractions better.  I’m going to pretty much keep what I got and be happy. I guess some people have approached my threads like I’m bragging on how much I spent.  I’m actually not and to be honest my system costs a fraction of the other guys I know.  Their systems cost 2,3,4 times what I paid.  I’m at a good spot in the hobby.  I think I have good enough stuff to pull a great sound that can surpass the way more expensive stuff 
For a quick answer:  Getting the bass correct.  I changed speakers over 10 times. Combinations with subwoofers didn't work. About 8 years ago I went with Evolution Acoustics MM2 which nailed it
For me its about the size of the soundstage, transparency (i.e. hearing the smallest details of the recording), and dimensionality.  I do think what type of music you are listening to is key.  I'm sure for heavy metal death rock there are other parameters one would search for, but for the classic folk rock I listen to, those are my goals.  All tube system with 45 SET monoblocks playing Dylan on vinyl, I can close my eyes and feel his presence in the room.  The better the system the more aware I become of how the source was recorder and mixed.  But I don't think $$ always equates with better.  My Cary equipment is marginally better than my bottlehead setup, but really it's mostly about aesthetics.  I've never heard an over $100,000 setup, maybe they would blow me away, but I can't help feeling that with the vibrancy of the under $5000 used market some people just have money to burn.  It's all good.  :-)
Teo_audio nailed it.

That's exactly what cheap electronic components are and what they sound like. My only quibble is this: the price spread in resistors is not 35:1, it's more than 1000:1 (nude Vishays retail at $16 each).

In my preamp, volume is controlled by a gain circuit, implemented as a rotary switch with discrete resistors. As an experiment, I put different types of resistor in each place, and heard huge differences, Vishay being clearly the best. Oddly enough, the most expensive resistor (yes, more expensive than Vishay) sounded the worst, and was unlistenable.

Same is true of capacitors. Only a few designers can make electrolytic caps sound like anything but nails on a chalkboard; but they are small, and cheap, and therefore popular with some manufacturers.

Ditto for mechanical noise. A cheap turntable introduces all sorts of noise which manifests as high frequency tizz. Why did that rube spend $10,000 on a turntable which sounds so dark? Because, on careful listening, it produces sounds similar to live music. Because you can listen for 4 hours at a stretch, and be sorry it can't be for 5.

Once you have heard these things, it can become an obsession, as it obviously did with me.