OK. 350+ posts in to this lot and after reading it all it's time, I think, to make myself unpopular.
Pacing: there is nothing wrong with a slow and detailed pace of play. If a campaign isn't set up to be open-ended in how long it'll last I'm probably not interested unless it's specifically designed to be a one-off; and if it's open-ended then it doesn't or shouldn't matter whether it takes 3 sessions or 19 sessions to get through a particular adventure as long as we're all having fun in the process. (ditto for level advancement - slow this down too) Here (and maybe only here) I agree with how [MENTION=97077]iserith[/MENTION] does it, in expecting/demanding a reasonable level of detail from the player in describing not only what her character is trying to do but how she is going about it. But that said...
DM assumptions: the DM has to be able to assume that barring unusual circumstances a PC is always using his or her senses in a manner sensible to the surroundings. This means, for example, a DM can reasonably assume a higher level of alertness from a PC walking down a passage deep in a dangerous dungeon than from one who is walking along a quiet country road toward a well-known village. The DM has to be able to assume the PCs are looking where they're going and paying at least passing attention to sounds, smells, movement, and so on occurring around them.
But the DM cannot assume what the PCs know or don't know or remember about things that might be obscure e.g. an old statue. Here it's not so much the DM who gatekeeps the knowledge but the dice, and somewhat-random determination of these things is perfectly reasonable.
A follow-on example from the statue: quite some time ago the party found a statue in a dungeon, had no idea who or what it was at the time but on getting back to town they made some inquiries and learned that it was a statue of Arevane, an obscure goddess of waters and seas. Now, fifteen adventures and two years later, they've found another statue. There's only two characters left in this party who were around when that first Arevane statue was found; so here I'd give a pre-emptive roll to those two characters* (without saying why or what it was for) to see if they remembered Arevane well enough to determine whether or not this new statue is another one of her. Success (either one): they know for sure it is (or is not) Arevane. Both fail: they've forgotten Arevane for the moment and thus don't have that information to provide.
* - or in a case like this I'll sometimes just use player memory and equate it to character memory: if the player remembers it, the character does.
Meta-telegraphing or cueing what's important: bad bad bad bad bad! When the characters look around an unoccupied room and see, let's say, seven different elements - a desk, a cabinet, some shelves, a chair, papers-pen-ink on the desk, books on the shelves, and a bottle of liquor on the floor by the chair - and one of those elements (let's say one of the books is hollow, hiding a key they need) is particularly important the DM should not emphasize the books any more (or any less) than any other element in the room. Why? 'Cause otherwise you're just leading 'em by the nose.
Same thing with telegraphing potential threats. Sometimes it makes sense, sometimes not. Thinking of the pickpocket example, sometimes there might not be an obvious threat to telegraph: "the town seems safe and the people happy as they go about their business" could be a valid part of an environment description of a town that has a thieves' guild who are very careful never to prey on the locals, but visitors like you are fair game...
Pre-emptively calling for rolls: as I said in another thread, nothing at all wrong with this. The example of spotting or not spotting a friend in the street is a very good one - here I'd give a pre-emptive roll but not tell the player why; I'd just say "<player name>, roll me a d20." (player name used instead of character name as we often run two characters, here I'm also hiding which character is being rolled for. If the player just has one character at the moment I'd use the character name)
On success I say: "Kirkos, as you pass through town you unexpectedly happen to see your sister Kirke across the street."
On failure I say: "Ignore me, carry on."
The other pre-emptive roll is for how much you notice. In the armour example with the red eyes, were it me I'd likely have got them to roll (hard to miss, but possible) before describing the hallway to see if they noticed the eyes in the shield were moving. Succeed: I describe it now. Fail: I describe it later if-when someone takes a closer look or when another pre-emptive roll (they'd get at least one more if they just walked past the armour) succeeds. Here the pre-emptive roll is more to determine when something is described, and-or in how much detail.
Saving throws for items and possessions: we do it every time someone fails a save vs. most damaging AoE effects, and if magic fails it might go up with a bang and-or a wild surge. Magic is both high risk high reward and easy come easy go; knowing it'll eventually get broken allows me to give out more, and players IME love finding new toys.
There is nothing wrong with gotchas. If there's a lurker above and you're not specifically looking up, well, that's what the surprise roll is for: you still might notice it anyway as it detaches from the ceiling and drops. (and yes there's going to be a surprise roll mechanic in any system I ever run, RAW be damned) And if avoiding gotchas means slowing down and paying attention and being cautious, then slow down and pay attention and be cautious.
The only trick is to remember gotchas can work both ways - if the PCs take the time to set up a gotcha for an enemy give the enemy the same honest chance to notice/avoid it as if that enemy was a PC, and if the enemy fails to notice it then let the PCs have their win. If a PC Thief wants to cruise the town and lighten a few pockets here and there it should be treated the same as if a local thief is trying to rob the PCs. And so on.
PCs missing vital information due to failed rolls and-or looking in the wrong place: an occasional fact of life the DM just has to deal with. Let 'em miss it, even if your storyline gets detoured or even goes out the window. Sometimes other ways present themselves for that info to reach the PCs, other times - e.g. the PCs fail to find a hidden map in a drawer and thus miss an entire secret level of the dungeon - they don't. So be it. Move on.
The alternative is to somehow lead 'em by the nose back to whatever they missed, which verges into railroading of the highest order.
Lan-"gotcha!"-efan