The progression is tiered, so if you have to work your way through the past tiers to get geared for the later tiers. Currently, my guild is working on the last boss of the 20 man Tier 2 raid Hammerknell (HK). There is a tier 3 raid out, Infernal Dawn, but we will have to keep running HK in order to get geared for ID. There are also certain parts of raids where you need resistances, which remind me of vanilla (though its much simpler and not as time consuming to get them).
Overall, I love tiered raids. I've been with my current guild since I started playing and it really helps you get into the group mindset when you have to earn the right to go into the later raids as a group. I regret not ever killing Heroic Ragnaros (WoW Firelands) because a new raid came out and no one wanted to go back and push through it.
There is "catch-up" gear but it's never enough to fully gear you out for the edge of content. Meaning it only serves the purpose of lessening the amount of grind from previous tiers. At a certain point... well, it's like TBC. Yea.