This tier list is based solely off drafting for SL. The tiers are based off how easy or hard it is to make the character work. Characters that can be first picked are towards the top; characters that need to be picked last are towards the bottom.
If you first pick a character, you give the enemy team five chances to counter you. Heroes at the top are either too difficult to counter in SL (because the solutions require co-ordination or mechanically intensive play) or not worth the risk of countering (why get three blinds for Greymane when they can take Uther).
Depending on what role you play, you may be better off picking "as late as possible". Of course you can rip an Abathur on the 1, but would you do so readily with Cho'Gall? .
If you're proficient on a hero, it's automagically a First Pick. How good you are at a character matters way more than what's 'relatively' powerful.
If you're not proficient at a character, drop it five tiers to the bottom.
If the enemy's got a one-trick, that character gets promoted to Ban Tier.
Finally, how much play a character sees does impact where they go on this list, but I'll still rate power picks very highly. I will always see Maiev as a ban-worthy character, even though few people play her; that sort of thing.
Heroes here have universal power; they're either very flexible through talents, they shut down a lot of heroes (closing options for opponents), or they're false flags that confuse opponents on the draft screen.
Heroes sit in second rotation primarily because they're not worth picking above more-contested characters. You can afford to wait on these because they're unlikely to be stolen from you. Alternatively, you need to see a few characters from the enemy team before you can judge them worth taking.
"Counters" generally sit here. So does most of the roster.
Heroes aren't here because they're bad, they're here because they're easily countered by multiple common characters. You draft them late to avoid that minefield.
Heroes left unchecked in this tier can absolutely dominate.
No character is "so bad you never pick it". There are players passionate about every hero in the game. You want your teammates on their comfort picks, no matter what that entails. You can make competence work - picking meta heroes does not magically inject skill.
That being said, there are certain characters that are pound-for-pound weaker than the alternatives. If you enjoy these characters and play them well, by all means, continue picking them. As I said earlier, the most important thing is competence; the tier list is just for drafting.