You know, I figured, what the heck. I have my own perspective on the Tiers, so I may as well share it.
ACG = Advanced Class Guide Playtest
Anything in red is weak for its tier. Anything in blue is strong for its tier. Not sure if there's a point to color-coding Tier 1s or Tier 6s, but I marked the vow of poverty monk in red for Tier 6 since it's so stupid it's usually in a league of its own.
Tier 1: Capable of doing absolutely everything, often better than classes that specialize in that thing. Often capable of solving encounters with a single mechanical ability and little thought from the player. Has world changing powers at high levels. These guys, if played with skill, can easily break a campaign and can be very hard to challenge without extreme DM fiat or plenty of house rules, especially if Tier 3s and below are in the party.
Wizard, Druid, Cleric, Witch, Sorcerer (Razmiran Priest archetype, Paragon Surge spell, Mongrel Mage archetype, Mnemonic Vestment robe), Oracle (Paragon Surge spell, Mnemonic Vestment robe, Dreamed Secrets feat, Spirit Guide with Versatile Spontaneity + Magical Epiphany feats), Psychic (Mnemonic Esoterica discipline power, Mnemonic Vestment robe, Amnesiac archetype), Shaman, Arcanist, Bard/Skald (Music Beyond the Spheres masterpiece)
Tier 2: Has as much raw power as the Tier 1 classes, but can't pull off nearly as many tricks, and while the class itself is capable of anything, no one build can actually do nearly as much as the Tier 1 classes. Still potentially campaign smashers by using the right abilities, but at the same time are more predictable and can't always have the right tool for the job. If the Tier 1 classes are countries with 10,000 nuclear weapons in their arsenal, these guys are countries with 10 nukes. Still dangerous and easily world shattering, but not in quite so many ways. Note that the Tier 2 classes are often less flexible than Tier 3 classes... it's just that their incredible potential power overwhelms their lack in flexibility.
Oracle, Psychic, Sorcerer, Summoner, Unchained Summoner, Monster Tactician Inquisitor
Tier 3: Capable of doing one thing quite well, while still being useful when that one thing is inappropriate, or capable of doing all things, but not as well as classes that specialize in that area. Occasionally has a mechanical ability that can solve an encounter, but this is relatively rare and easy to deal with. Can be game breaking only with specific intent to do so. Challenging such a character takes some thought from the DM, but isn't too difficult. Will outshine any Tier 5s in the party much of the time.
Alchemist, Bard, Skald, Inquisitor, Magus, Investigator, Warpriest, Eldritch Scoundrel Rogue, Vigilante w/ spellcasting archetype, Occultist, Mesmerist (when it can seance its spirits easily), Medium, Spiritualist, Hunter w/ Divine Hunter + Primal Companion Hunter archetypes, Sacred Servant+Oath of Vengeance Paladin, Fiendish Bond Antipaladin with Succubus or Shadow Demon
Tier 4: Capable of doing one thing quite well, but often useless when encounters require other areas of expertise, or capable of doing many things to a reasonable degree of competence without truly shining. Rarely has any abilities that can outright handle an encounter unless that encounter plays directly to the class's main strength. DMs may sometimes need to work to make sure Tier 4s can contribute to an encounter, as their abilities may sometimes leave them useless. Won't outshine anyone except Tier 6s except in specific circumstances that play to their strengths. Cannot compete effectively with Tier 1s that are played well.
Adept, Barbarian, Unchained Barbarian, Bloodrager, Archetyped Brawlers, Paladin, Ranger, Slayer, Martial Master and/or Mutation Warrior Fighter, Mesmerist (when it can't seance well), Kineticist, Hunter, Shifter, Antipaladin, Sohei Monk (preferably with Qinggong Monk powers)
Tier 5: Capable of doing only one thing, and not necessarily all that well, or so unfocused that they have trouble mastering anything, and in many types of encounters the character cannot contribute. In some cases, can do one thing very well, but that one thing is very often not needed. Has trouble shining in any encounter unless the encounter matches their strengths. DMs may have to work to avoid the player feeling that their character is worthless unless the entire party is Tier 4 and below. Characters in this tier will often feel like one trick ponies if they do well, or just feel like they have no tricks at all if they build the class poorly.
Fighter, Vanilla Brawler, Vigilante, Ninja, Rogue, Unchained Rogue, Cavalier, Samurai, Gunslinger, Swashbuckler (ACG), Archetyped Monks, Unchained Monk
Tier 6: Not even capable of shining in their own area of expertise. DMs will need to work hard to make encounters that this sort of character can contribute in with their mechanical abilities. Will often feel worthless unless the character is seriously powergamed beyond belief, and even then won't be terribly impressive. Needs to fight enemies of lower than normal CR. Class is often completely unsynergized or with almost no abilities of merit. Avoid allowing PCs to play these characters.
Vanilla Monk, Aristocrat, Expert, Warrior, Commoner, Vow of Poverty Monk