I have to completely disagree with you about T5 being compared to A3 or A4. Sorry, but no.
The mechanics of A3 and A4 are a joke. A4, in particular, is pathetic and brings nothing new to raiding. Break it down and you'll see: 1. soak dmg (orbs). 2. soak more dmg (tethers) 3. Kill adds. 4. kill more adds (in tunnel). Repeat. They changed the way some of these mechs look (getting sucked into a tunnel is cool and never gets boring), but they're nothing we haven't dealt with before, multiple times, and under much more difficult conditions. By comparison, there is nothing in either of those fights that are even remotely close to being as difficult as Twisters/Divebombs/dreadknights in the T5 fight back when i90 was the highest gear level.
As for the "feeling" of the raid, I think the Goblins are legitimately the problem. There's a lack of intimidation and mystery in Alex by comparison to the Coil. I don't think that there was a single player who walked into the Coil, after the first boss in T1, and saw Bahamut's wing and wasn't completely awestruck by the gravity of what they had gotten themselves into. We didn't really know where the coils were going (who summoned him, why, how, why is he in pieces, is he awake, will/when will he start moving... etc.), but there was always this crushing sense that we'd gotten ourselves in over our heads. It made you paranoid and constantly on guard, but you also didn't want to stop.
With Alex, we know exactly what we got ourselves into. The Goblin's summoned him to create their dumb utopia blah blah blah... If they sent us in there with no back story and the Goblin's weren't around, I think it would have been more tense. We'd be wandering through an iron giant we knew nothing about, were never sure if it was gonna come alive, or what kind of things were hidden in it's body. Right now, we know pretty much everything we need to know and that's why there is no impact. We're doing a job, not solving a mystery, and by comparison that's boring.