halacha theory - What's so slave-ish in the Hebrew Slave?

I'm trying to compare the status and duties of a Hebrew slave that sells himself to slavery versus a man that agrees to work for 7 years (שכיר שבוע), as in B"M 9,11: "שְׂכִיר שַׁבָּת, שְׂכִיר חֹדֶשׁ, שְׂכִיר שָׁנָה, שְׂכִיר שָׁבוּעַ, יָצָא בַיּוֹם, גּוֹבֶה כָל הַיּוֹם".

Here are some points of similarity:

1. They both don't work on Shabbos and Yom Tov


2. They both own their property, and their body is not their master's property


3. While at work (practically for all the time) their profits (incl. lost and found) belong to the master


4. They stay married with kids


5. They stay obligated in all Mitzvos


6. All negative commandments toward one's fellow Jew hold for both (Ona'a, beating etc)


7. The master pays money to both

Here are some points of differences:

1. Master pays in advance vs after the work


2. Master can't force him to do a disregarding job, one that will "reveal his slavery"


3. Master has to equal his conditions to his own


4. Master is obligated to pay him some extra on freeing, and some more.

The last list in comparison to the first does not justify, in my opinion, the "essence of slavery" - to call this Jew a slave, especially in comparison to a real slave - Eved Knaani, whose body is his master's property!

As usual, unfortunately, I couldn't find an official definition of a "slave" - what qualities make one a Hebrew slave. Hence the question: why a Hebrew slave is called really a "slave" and not, for example, a persistent/diligent/enduring employee?