Tony Vargas
Legend
Monsters could also have PC classes, and monsters that could cast spells cast the same spells as PCs. I don't think that exactly maintained a 'curtain' between the players and the mechanics. Rather, the mechanics were right there, to be understood and exploited. The best way to exploit the mechanics, though, defined the way things work in the game. That gives the game a 'right' (or best) way to be played and the world a definite feel.3.x monsters are built the same as 3.x PCs, sort of. Each monster type is a sort of "class," with a HD type, a BAB progression, saves and so forth.
I can see how that might 'break immersion' - for the DM, since he's the only one dealing with monster design. IMPX, once I GM a game, that sort of immersion in that game is dead for me, because I'll reverse-engineer and tinker with the rules and generally run the darn game, which definitely means looking behind the curtain no matter how thick or stapled to the floor it may be, or how ugly and inefficient the machinery behind it. So, as a DM, I'll take a gauzy curtain with elegant, smooth-functioning, easy-to-maintain machinery behind it, thank you.4e drew the curtain back, and drew attention to the hardest part of monster design: challenging the players and PCs. ... Hence for example the shift from CR, which is just sort of thrown onto a monster after the fact, to monster level, which is one of a 4e monster's most basic stats that other stats are based on.
But as a player, monsters working by their own formulae, which are different from your own, means each monster gets to be different, and each monster power doesn't work exactly like some spell you have. That shouldn't hurt 'immersion' at all.
Actually, now that I think of it, the best 'immersive' experiences I've had with D&D have been early in a give ed, when I haven't digested everything yet and am just playing. Not knowing all the rules about the monsters, the other PCs, and so forth does give you that sense of being in a mysterious or fantastic world.
However, that doesn't map precisely to having radically different rules for everything. Just to being able to concentrate on the rules you need to play your character to the exclusion of the details of everyone else's & the DM's. In classic D&D, if you only ever played one class, you could retain that blissful ignorance, for a while, until it just became obvious what other classes could do. Same with 3.x, though, once you played /any/ caster, you were unlikely to make it through the task of understanding and picking your spells without noticing what a lot of other casters could do (just because of how the spells were organized, all mixed together and shared rather than class-by-class as in AD&D). In 4e, you could focus on your character, and not even have to really learn all the powers of your class - just look over the choices at a given level once, pick one, and really learn how to use that power - not being able to duplicate (and thus spam) more potent encounters and dailies also helped with that, a bit. For me, that meant the 'immersive' experience lasted through several characters in 4e, vs just one (though it was a lot of fun) in 3.0.