So, I've frequently referred to Pathfinder and Wizard's of the Coast's versions of D&D (WD&D) as "too fiddly" for me to ever want to run.  I'll play in them, sure, but there's a couple meanings in "too fiddly" that I want to talk about.

1)  Character creation as a winnable mini-game.  In AD&D and C&C, you create a character of X race and Y class, at Z level.  For the most part, that mechanically defines your character.  Unless you're a human dual-class in AD&D, you're not going to significantly change beyond that from level to level... you'll learn new weapons, and new spells, and various numbers will improve, but you're essentially the same set of mechanics throughout.  For me, this frees me to think about that story and interactions of the characters, both in character and out of character, and to make a mechanically ineffective character, you've more or less got to make a thief with a low dex, a wizard with a low int, or a cleric with a wisdom below 13.  Beyond that, you're going to be at least moderately effective, unless played poorly.  The chink in this part of the argument is, of course, Skills and Powers.  However, it is not as bad as you might think.  While it allowed each character vary quite a bit, the characters did not appreciably mechanically evolve from level to level.  Fighter A may have a variety of abilities that Fighter B does not, but Fighter A's variant abilities will remain largely the same across levels.  Aside from new spell levels, the only truly game-changing ability that happens in core AD&D is when the druid can finally shapechange (or the heirophant stuff, which I have never seen in play).
In 3.5 and Pathfinder, however, that's just the beginning.  Once you've chosen X and Z, you have to choose multiples for Y, and a myriad of choices.  What's your Y for Z1?  For Z2?  Are you going to change to something else at Z6, and if so, what choices do you have to make at Z 1-5?  In these two games, it is very possible to make a character who, though they seem interesting, are mechanically unfeasible.  If you mess up in one of your choices, you can screw yourself out of a prestige class, or even wind up preparing for one that doesn't really help you do what you want to do.  There's a dizzying array of options, and some of them are simply mechanical traps... they work poorly or not as well as some other option that you overlooked (or was in some splatbook that you didn't have).  The phrase "build" comes into play because that's what you do, level by level... mechanically put together your character, making changes with each level so that, for me at least, there's a space of 2-3 levels where the character is like I pictured him... and then he's changed to something else, because of mechanical choices that came up.
4e tries to fix this, and does, to some extent.  While you usually have at least one choice at every level (feat, power, path or attribute), most of them are relatively balanced with each other, and it takes a little bit of work to make a mechanically unfeasible character (i.e. putting your highest stat as something other than your attack... or at least secondary... stat; maybe choosing feats that will be absolutely useless to you, or one of the powers that simply does not work well).  While a new level in 4e seldom radically changes the capabilities of your character, it is still a game that places a fair amount of emphasis on your mechanical build.

2)  Overall mechanics.  In 3.x, Pathfinder, and 4e, you have a plethora of bonuses and situational modifiers.  "I was bloodied this turn, so I get a +2 to damage, and I spent an action point so I get +2 to every die of damage... I can't stack Paragon Defenses and Iron Will because they're both feat bonuses, but I can stack these three because one's an armor bonus and the other's a natural armor bonus and the last one's an enhancement to my natural armor bonus... but I don't get that one on this attack because he has combat advantage until I save against it."  In both my Pathfinder games and my 4e games, a frequent lament is "Crap, I forgot I had this bonus, I probably would have (saved/hit/killed him/not died).  Round to round, even with combat cards and the like, this becomes a LOT to keep track of at once, especially if you're DMing.  You have to trust that the players know their characters very well, because each is a special and unique snowflake that, despite using unified mechanics, has completely different sets of modifiers to keep track of... as do most of the monsters.  This is still a mountain of information to juggle at any given moment.
Compare this to AD&D or C&C.  Modifiers tend to be fewer, and they tend to be more standardized across classes.  While this slightly narrows tactical options (and, IME, it is only a slight narrowing), it keeps game play much faster... with fewer tactical options, players dither shorter periods of time, and calculate effects MUCH faster when they don't have 4-5 different numbers and modifiers to keep track of.  Especially with AD&D, you do have different mechanics being handled with different systems... but those systems, once learned, remain the same.  There's not a point where you have to change how a surprise check is done because of a new class ability, or find yourself facing an ever-changing array of modifiers.

Despite AD&D's convoluted mechanical eccentricities, I maintain that it was inherently simpler than Pathfinder, 3.x, or 4e.  Once the mechanics of the system were learned, they could be subsumed; their few modifiers either learned or placed for reference.  With WD&D and its successors, you are instead presented with mechanics that may revolve around a central, unifying die roll... but where each roll is subject to a unique set of modifiers, from many different sources (PCs, NPCs, environment) which are not so easily memorized because they frequently change with each roll.  This does not say that AD&D is "better" than the other games... that is, in many ways, a subjective measurement... but it is simpler, especially on the DM side of the screen.