You're free to hold your own beliefs. A lot of what we're discussing, as one poster said, is theater of the mind stuff. I'll respond to a few things:
As Infinitron said, Irenicus is thrown in your face in the prologue in all of his power and "evilness", Rich Homie Cloaked's all underlined and emphasized to the max
In point of fact, ambiguities abound with Irenicus. At no point does he treat you as a threat or an enemy in the tutorial area. You're an item, of sorts, and he's figuring out your functions and parameters. Certainly cold and calculating (the dialogue bears that out), but not some chaotic evil buffon just out to main and murder. There's clearly method and
purpose to his madness, and the game makes you want to find out what that is by upping the ante (rescuing Imoen/personal vengeance). The reason why this is pertinent is because his motivations are - at that stage - totally unclear. The whole dungeon abounds with strangeness. From the creatures in the stasis tanks, to his personal chambers and the nymphs, there's a lot of mud in the water. You don't even begin to understand Irenicus until well after Chapter 2. His display of power establishes his pedigree and, in terms of drama, does it far better than Sarevok's murder of Gorion. Especially because the latter brings his three minions along as cannon fodder. The writers show Irenicus to be an incredibly powerful mage; supremely confident in his own ability
and arrogant to the point of foolhardiness. They do it in a few short lines, relying on David Warner's presence and the fireworks on display.
Sarevok swings his sword a few times and says nothing of import... He's a suit of armor with a deep voice.
Then you spend dozens of hours uncovering the double-layered plot, first Rich Homie Cloaked seems Iron Throne is behind all the problems and Sarevok is just their top henchman, before the reveal that he has manipulated Iron Throne as well.
Well, you can't simultaneously say (not meaning you, specifically) that Irenicus is out of sight and out of mind for hours and hours as a criticism, and then say the above about Sarevok. The fact is, you can go those dozens and dozens of hours totally unaware of Sarevok on any immediate level. He becomes an issue in, like, the last two chapters (from memory).
If you do a side by side comparison of how much integration the two villains have into their respective game areas, BG2 blows BG1 out of the water. Irenicus is woven in and out of the narrative seamlessly. You're encountering his minions, his sister, his left overs etc. throughout the game. Sarevok's minions are around, but they never feel connected to Sarevok, nor are you given a reason to feel that connection.
In terms of the writing, this is a total non-contest. To me, Irenicus is one of the greatest video gaming villains ever. You guys certainly don't have to agree with me.
those who like him do so for the expert voice acting..
VA is part of it, but the real reason is that you can actually see decent writers honing their craft in BG2. Sarevok and BG1 was Baby's First Heroic Journey.