Everyone here already knows how I feel about Glavenus.
Design wise it's one of the least changed returnees in Iceborne. Its scales and shells do look more detailed but this is hard to notice because of two reasons: It came from a very recent title (rather than being a model ported over several generations), and it's an incredibly busy design to begin with anyways. The changes during rage mode are now much more subtle, which works in my case since I've never liked the spikes turning red. The heated tail actually glows in dark areas or at night which is neat.
The pattern of "there's nothing new" continues when it comes to the gamplay. Glavenus is (alongside Diablos and Rathian) the monster that changes the least from its previous incarnation, not only for Iceborne but World as a whole. The number of new attacks it gets adds up to the grand total of 1: One half-new move where it makes a small sweep to the side after its tail slam, and another half-new move where it flinches and does a counter bite if you do enough damage while it readies its roundslash. Its fireballs also received a nerf in that the explosion that occur after are now MUCH smaller, but this is one change I actually don't mind since Glav had no business making 10 feet fire pillars erupt from the ground like it was a breath-based monster. Something it also lost is its throat heating, it will now always heat it up as it sharpens its tail, rather than being two separate things. I guess this was made so it's not as similar to Anjanath, which is pretty stupid when Anja was the one to steal that gimmick in first place. Either way, everything else is identical, exact same attacks, exact same animations, exact same counterplay. The only exception to this is, of course, Glavenus speed and its ability to chain attacks. Or rather, lack of thereof. Glavenus is twice as sluggish as it was in the orevious two games. What makes for yet another case of a low rank monster masquerading as a G-rank one in Iceborne. Narga was almost passable because it at least got its fair share of new moves (emphasis on almost). With Glavenus, it takes you maybe a couple minutes to internalize it's not gonna follow up on anything, and then it's the exact same fight as your village urgent in MHGU. Needless to say all this makes Glav incredibly boring, but there's still one more thing that makes it even worse. Glavenus is tanky. Way more than it needs to be. Its hitzones were buffed (or nerfed? I don't know which term is more fitting here) across the board. Naturally, you already know why: To make way for the super duper cool, not forced at all, new revolutionary way of hunting: the Clutch Claw. Non-weak parts that were more lenient before now take almost no damage, and even weak parts like its head or tail need softening first to trigger skills like Weakness Exploit or deal meaningful damage. Glavenus has become an exponentially much worse fight than it used to be, which is honestly impressive given how it's almost a straight port from GU.
Now, despite the plethora of issues I might have had with the original Glavenus, let me assure you the sound department was not one of them. Glavenus' theme was fantastic, but its remix in Iceborne traded all the intensity and personality of the theme for mediocre synths and terrible progression. Gone are all the sword clashing-like effects, now replaced by orchestra as if instead of Monster Hunter this were Jurassic Park. To fill space, the main verse is repeated ad infinitum until it suddenly comes to a halt, trying to emulate the original's drop except with nothing of the energy it had. It's like it's trying to be Deviljho v.2 rather than its own theme. By far the worst remix in Iceborne. For its roars, it's a bit of a strange situation. It's not bad because they completely fail at reproducing what the monster is supposed to sound like, like Barioth or Garuga. Although it has a couple of odd sounding whimpers and it sometimes does N64 Bowser's roar, Glavenus sounds largely the same as it did in Gen 4. Rather, it's bad because, although it keeps the same roars, those are, for some incomprehensible reason, much more diluted. Basically, this dino is aphonic. The move I like to use to exemplify this is its roundslash, since it perfectly encapsulates my problem with this. The move, in 4th gen, had three distinct sounds; first, an unmistakable, prolonged metallic sound as it grinds it tail on its teeth to tell you the big attack is coming. Second, the sound of the blade cutting the air as it spins, and finally, a triumphant roar. In Iceborne, all of this is replaced with absolutely nothing. Just the deafening sound of silence.
Not much worth to note when it comes to the Equipment. The armor looks like it's, appropiately, trying to be a GS set, but it falls under its weight mainly because >Max Might in Iceborne. Also you wouldn't even want to go for aesthetics and pair it with its GS because, out of all weapons, they chose to IRON that. The Great Sword monster doesn't even have its original design Great Sword? It's like you're doing it on purpose at this point, Capcom. I have never used Glav's weapons, so I can't say much for their viability, although I think I remember watching a run using Glavenus Bardred, which probably means it's a fairly strong elemental CB.
If I was already questioning why they decided to bring Glavenus back for Iceborne, those feelings have only become stronger after seeing it my myself in the game. At least, it can continue to serve its purpose as the biggest pleb filter in the series. As in, if someone says "I think Glavenus is good!" I can immediately disregard and ignore everything that comes out of that "person's" mouth. I guess I can can appreciate it for that.