I have felt a bit lost in my direction lately - probably my own fault as I am always wandering off to do something else. :) So I decided to do some work on armor progression from level 70 and up.

Please do offer your input - corrections-and feedback. I plan to edit this post as I experience and learn more.

I also want to make mention of AA. AA can make a world of change in your game play. It is easy to level, but can take longer to get AA.

Tier 8 armor gets complicated because level 77-80 covers 2 expansions, Rise of Kunark and The Shadow Odyssey.

In a nutshell:

Rise of Kunark
The Shadow Odyssey
Legendary (TSO T2 void shard armor)
The Shard of Hate (SoH)
The Great Divide
Forgotten Pools.
Tower of Frozen Shadow
Rime - The Ascent and The Fortress Spire
Outer Kael and Kael Drakkel
Ry'Gorr - Eastern Wastes

And for a more raid directed listing:

Getting Started

At level 80, before you get into raiding, your checklist of things to do should include:
• Acquire your Epic (Fabled) Weapon -- this is a quest that has some steps that require a solid group
• Get a full set of Legendary armor -- these drop in Kunark, or better yet, run TSO dungeons to get enough Void Shards to buy the TSO T1 and then T2 Void Shard Armor. This will be supplemented by jewelry, etc. that drops from dungeon mobs, named mobs, and as quest rewards.
• Full Adept III spells - of course, Master is always best.
• Acquire 150 AA points or more. This is through questing, discovering new areas, and killing named encounters.

Rise of Kunark
The Rise of Kunark has a raid progression.
Kunark T1 Raids (Pawbuster, Tairiza the Widow Mistress, and Protector's Realm) are zones which will test your raid force's ability to cure/heal, DPS, and coordinate.
Kunark T2 Raids (Venril Sathir, The Temple of Kor-Sha, the Leviathan) are zones which require an intense amount of coordination and discipline in your raid force.
Kunark T3 Raid (Veeshan's Peak -- the 12 Dragons) is a zone that tests everything you learned in T1 and T2 Kunark raid zones.
Every member of your raid force must have cleared the boss of T2 (the Leviathan) to zone into Veeshan's Peak. There are dozens of raid forces which have cleared Veeshan's Peak and moved onto (and cleared) the final boss of Kunark (excluding Gods and SoH) -- Trakanon.

The Shadow Odyssey
The Shadow Odyssey does not have a clear raid progression. You can walk into any raid zone in TSO in any order with no prerequisites. Each of the 3 raid zones in TSO progress from medium difficulty up to the point where few raid forces have cleared them.
There are four TSO x4 raid zones now. They are adding a fifth x4 raid zone and a new x2 raid zone in the next game update.
Other Zones
Between the release of The Rise of Kunark and The Shadow Odyssey, a new raid zone was added which falls outside any previous progression. The Shard of Hate (SoH) is a large outdoor raid zone which is open to all and ramps up in difficulty from easymode trash mobs (which can be done with as few as 6 exceptionally well-geared players) to one of the hardest encounters in the game -- Byzola.

Kunark Armor Progression
Mastercrafted is your basic level. There is level 72 and some level 77 mastercrafted. Kunark dungeons are doable with it. You will die almost instantly in TSO dungeons with it.
Legendary (Kunark quest rewards, trash mob drops, etc.) -- some of it is better than MC (mastercrafted), most is worse.
Legendary (Kunark level 77 armor sets) -- This used to be the Legendary armor set that most players had at level 80. An example item is Spaulders of Ravaged Onslaught. This is what people started the Kunark Raid Progression wearing. These Legendary sets were quite limited as they only had 1 set for all Mages, and only 1 set for all Scouts.
Fabled (Kunark raid zone drops) -- As you make your way through the Kunark raid zone progression (T1, T2, T3/Veeshan's Peak) you can start gearing up on raid gear necessary to complete harder content. The TSO T2 Void Shard armor makes this Kunark raid progression a lot easier, but not trivial by any stretch. People still have to know their classes.
Fabled (Kunark Veeshan's Peak fabled pattern armor) -- Bosses in Veeshan's Peak drop armor patterns and gems which are used to make VP fabled pattern armor. This was at one time the best armor in the game outside of Gods and SoH.

The Shadow Odyssey Armor Progression
Legendary (TSO T1 void shard armor) -- The Shadow Odyssey introduced 20 dungeons, 17 of which reward players with Void Shards. Void Shards are redeemed at merchants for Legendary Void Shard Armor. For nearly all classes, this completely blows away Kunark legendary sets.
Legendary (TSO T2 void shard armor) -- At first glance, these appear to hardly be any upgrade over T1. However they contain a tiny bit of an important stat - Critical Mitigation - and provide boosts to almost all the other stats and bonuses on the T1 armor. This is considered the baseline armor necessary to do any TSO Raid Zones.
Fabled (TSO x2 Raid Zone shard armor) -- In the next major game update, a new x2 raid zone will drop Molds and void shards which, when combined with TSO T2 void shard armor, give you TSO T3 Void Shard Armor (Fabled).
Fabled (TSO x4 raid zones pattern armor) -- TSO launched with 4 raid zones (a 5th one will be added next update). There are bosses in these zones which drop patterns necessary to make TSO Fabled Pattern Armor (some people are now calling this T4 armor). It should be noted that this armor took a direct path from the Kunark Veeshan's Peak raid armor and has similar bonuses and statistics.
Fabled (TSO raid zone drops) -- Of course each of the TSO Raid Zones also have treasure chests with various upgrades. Some are better than the pattern armor.

Miscellaneous Armor
Shard of Hate (added between Kunark and TSO) -- This zone is an anomaly. It was added between expansions and does not share the lore or storyline of either. Also, the itemization has been accused of being overpowered and it does not really form a smooth progression between anything that came before it or since. Some of the SoH items are still the best items in the game for certain classes. As this zone ramps up in difficulty from easymode to the hardest content in the game, it became a very popular place and it is still visited frequently by beginning and advanced raiding guilds alike.
Avatars -- These are physical manifestations of the Gods of Norrath. Killing them requires an elite raid force with utmost discipline. These were revamped with the launch of TSO, and then revamped further to require TSO Void Shard Raid Armor. The item drops from these are tremendous. One Avatar drop can have more Critical Mitigation than an entire set of T2 armor.
As you can see, there is no direct progression through all of that. I will say for most situations, The Shadow Odyssey T2 Void Shard Armor will give you a leg up in almost all Kunark dungeons and starting raid zones. Then you hit a fork between Kunark raid gear and TSO raid gear and will probably want items from each to handle different situations. The most glaring example is the Assassin/Ranger TSO void shard armor set which does not have a single point of Divine resistance, which is crucial for some Kunark raid zones. So you may need to gear up some in Kunark first.

Suggested Armor Progression
If you have Mastercrafted now as a level 80 character, then you have a lot of work ahead of you. Run the TSO void shard zones until you have enough shards -38- for T1 and -112- for T2 assuming you have a void shard armor crafter in your guild (any tradeskiller who has done certain quests and gotten certain recipes can make them). Then if you were in a raid guild or were to join one, assuming everyone is just starting out, you would be tackling both Rise of Kunark raid zones to start working on your Mythical Weapon quest, and also starting to make your way into The Shadow Odyssey raid zones for loot and armor upgrades.
Critical Mitigation
You asked about Critical Mitigation. Hopefully you've noticed that your gear has Melee Crit, and other gear has Spell Crit, Ranged Crit, and Heal Crit. Starting with The Shadow Odyssey, the harder bosses of dungeons and raid zones also have these stats. These can cause devastating attacks and self-heals which make the bosses nearly impossible to defeat. Armor upgrades which provide Critical Mitigation will reduce these Crit percentages off of bosses so you can defeat them. Some classes can temporarily reduce the Critical modifiers of bosses.
And finally, some of the Kunark raid progression requires certain Resists and certain types of gear. Going straight for TSO armor and completely ignoring the Kunark raid zones, group zones, etc. would be a mistake because then you may not have the necessary Resists, Bonuses, Effects, etc. Also, you must complete certain Kunark content to get your Epic Weapon which leads to your Mythical weapon.

So...now you are level 86...
Head to The Great Divide and start with the quests on the docks. You will get a full set of armor and jewelery, plus coin and AA.
Onward to PQ's. Yes...you have to do a lot. Yes..the RNG hates you..Yes, the BP and weapon that you want will take forever to get, but you will increase your AA (if you are not already capped), you will meet folks, and you will notice an improvement even when you have 5 of the 7 pieces!
While you continue to try for those PQ's pieces..start running Forgotten Pools.
Tower of Frozen Shadow
Rime - The Ascent and The Fortress Spire
Outer Kael and Kael Drakkel
Ry'Gorr - Eastern Wastes

Edited to add very nice link to EQ2Wire Guide to Velious Armor (thank goodness!):