How does mainstream Christianity's perception of Heaven differ from the Mormon perception of the Terrestrial Kingdom? Unless I am mistaken, it seems to me that the Mormon definition of the Terrestrial Kingdom (the second highest kingdom of glory) is essentially identical to mainstream Christianity's definition of Heaven: 


It's a paradise
You can see Jesus there (just Jesus, he's the only God there)
You get to see all your deceased loved ones
You can't become a god


Mormons believe that the Terrestrial Kingdom is where Christians who do not accept the fullness of the Gospel (modern prophets, the Book of Mormon) will go after judgement:


  75 These are they who are honorable men of the earth, who were blinded
  by the craftiness of men.
  
  76 These are they who receive of his glory, but not of his fulness. (D&amp;C 76:75, 76)


If this is true, then that basically means that Mormons believe all the other Christians are going exactly where they believe they are going. And Mormons simply believe in a higher heaven where you can be exalted and become a god, and that not being admitted into this higher heaven would be a type of Hell to them. 

Identities of deities and other doctrines aside, after judgement, how does existence in Heaven according to mainstream Christianity, differ from the glory of the Terrestrial Kingdom?
I want the answers to focus on the day-to-day living/quality of life/level of perfection in heaven.


That's next to impossible, as there aren't any detailed (canon) claims of day-to-day activity, quality of life, etc. that could help us out here.

"What is heaven like?" is one of the great mysteries. It probably has a similar answer to "What is being alive like?": you'd have to be there to know.

Comparison is even more tricky.

"Christians say that they will be happy by 1 unit." "Mormons say citizens of the Terrestrial kingdom will be happy by 2 units." "3 units." "4 units!" "Infinity!" "Infinity + 1!"



Ostensibly, yes they are quite alike. There are many similarities as you pointed out.

Probably the biggest difference between mainstream Christian heaven and the LDS terrestial kingdom is this: mainstreams Christrians in heaven are as happy as they could ever possibly be. Those in the LDS Terrestrial kingdom aren't.