It wasn't any different or more interesting than Kings, Sky and Dynamis. Roc, Serket and that shit is akin to Great Buffalo being end-game content. That's like if Kings (a part of end-game that is widely despised) were the only thing you could do until CoP came out. Not to mention those monsters had limited loot pools with low drop rates with situational items (like everything else), so there's a limit to how lucrative it even was as an end-game activity.
And dynamis was the most boring, repetitive, soul sucking, long-winded event that was so clearly designed to suck all your time away I wonder how you can say that is truly good content. Sure, every 3 days you need up to (and I say up to because eventually it was reduced...eventually) 64 people to clear an event. Not to mention that for the most part, if you didn't get AF from it, you weren't getting anything so you had to spend 3.5 hours doing boring shit with no direct incentive. Take the blinders off and you'll see that dynamis is to content as piercing your dick is to pleasure. So, what did you do in-between dynamis? Sky, or if you were capable and the timing was right, Kings. I don't need to go into detail about Kings, but Sky was cool. Just a shame there was so much farming involved to get to decent boss fights.
None of these events could be done quickly or freely, and until CoP came along as a much needed distraction from the monotony, XI's end-game was just a cycle of 3 things that each took hours to complete every time. Then CoP came along, with staggered releases of Sea (CoP mission locked), and Limbus, but I remember until those came out everyone was just doing the same old thing. Then Limbus came as a welcome distraction to dynamis (and only because it was 10x shorter, not 10x more fun, though I did like fighting Omega and Ultima more than anything in dynamis) and Sea was an unwelcome distraction to Sky (seriously, Sea sucked). So the monotony continued, with only dragons (another large scale hassle-filled event) to pace out the monotony of Sea and Limbus.
Incase you're thinking at this point, "Boy, that sure is a lot of content!" keep in mind how boring and frustrating dynamis, sky and kings were already. Going back to old content is never fun and it wasn't in XI, and going forward to "new" content that was basically rehashed old content with new shiny gear just made for the worst end-game for ages. Eventually, it became monotonous because it never truly was different to previous content. Just dressed differently.
Then ToAU came out and things got better. Assault and Salvage were good, still long-winded as hell and basically rehashing concepts from dynamis and limbus, we got more events and new things to do, but people still did old content. And it still wasn't fun. But it was still kinda relevant, so that sucked. But at least you had a lot of things to do, right?
No, not really. Unless you're easily excited by doing things you've done 1000 times before, you realistically only had a few things you could actively do at a time depending on your LS and your personal pain tolerance.