In ranked, people with similar mmr will get matched (at least in season 2).
In unranked this is not the case.
It can basically match the person with the lowest mmr and the highest mmr together in a game.
Combined mmr.
Unranked looks at combined team mmr (the mmr of all teammembers combined) and will try to get similar combined mmr in the other team.
Example:
Team A.
Team member1: 100 mmr.
Team member 2: 25 mmr.
Team member 3: 75 mmr.
Team member 4: 200 mmr.
Team member 5: 100 mmr.
Combined mmr: 500.
(The numbers could be different. As mmr is hidden. I am just trying to give an example of possible gaps).
Now it will try to get 500 combined mmr in team B too.
People with completely different mindsets, intentions, goals, skills etc. get matched in unranked with and against each other.
This obviously gives issues.
Examples:
-Someone who doesnt care about winning, who rarely pvps (low mmr), who just wants to run at some people and fight
and someone who wants to play seriously and try to win (lot higher mmr usually),
get matched together.
-Someone really experienced and skilled with high mmr and someone new to the game playing his first few matches (low mmr) get matched together.
This obviously gives issues.
-You dont want to get teamed up with people who dont try to win, when you do.
-People dont want to get owned by someone way out of their mmr range and skill lvl (that is discouraging for new pvpers as well).
-People genenerally dont want to win without challenge either.
More pleasant.
It will be a more pleasant game experience if you make the mmr gaps between teammates less wide in unranked.
That will increase chances that people with more similar mindsets, intentions, goals, skills etc. will be matched with and against each other,
which gives a more pleasant gaming experience.