Ok, I could offer a whole bunch of tips here but I don't want to piss anyone off.  So I'll make a couple new posts with some hints to answer a couple not so obvious things that I found most difficult... but anyway... my non spoiling impressions:
 
First and foremost, this is Thief.  Plain, simple.  
I got off to a rocky start but I have to say LGS would be proud.  It feels incredibly like Thief right from the beginning.  I really didn't like Deus Ex 2 as a game and feared the worst for Thief 3, but I was wrong and I apologize to ISA for all the bad things I've been thinking.  
Also to Greger, who tried endlessly to set me straight but I was thick headed and wouldn't listen.
 
COST:  $40 USD.  I've gone through the training and the first mission.  This game is worth every penny as of right now.  
Can't wait to see the rest of
it.
 
TRAINING: Training was great.  
I enjoyed it a lot more than T1's training.
 
ATMOSPHERE: Same kind of ambient hum as T1/T2.  Very similar feeling, much better graphics.
 
GRAPHICS: I'm running 1024x768, no problem.  I have no fancy sound settings on though, which I've seen posts elsewhere complaining about their framerates with it on.  I have it off and it's all good.
 
FEEL/CONTROL: It is a little jerky at times.  Where you try to turn or stop running and it just feels a little awkward.  I haven't adjusted to it yet, so I don't know if it's something you can completely get used to, but I'm really stretching to be nitpicky here.  It's hardly noticeable and certainly doesn't take me out of the action when I do notice it.  It's kind of like walking something on the ground that you can't see, so Garrett tries to step around it or something.
 
MANTLING: Mantling is very smooth.  It will also take a bit to adjust to it, but it's much cleaner than T2.
 
CROUCH-DROP: So far I haven't been able to crouch-drop from a height and not make a noise.  
But I've only had the opportunity a few times, so it might just be a learned technique.
 
LOCK PICKING: Lock picking is really sweet but you have to understand a
fundamental rule.  I'll post that rule in a later post so as not to spoil anything for anyone.
 
RUNNING: You run by default, which kind of sucks but not really.  I usually dash through the house anyway and slow down when I hear a guard.
 
LEANING: I think I read somewhere there's no leaning.  It's in there though, so stop lying
 
FIGHTING: Yeah, just give it up, fighting ain't pretty.
 
BOW/ARROWS: All I can say to Greger is HA HA HA!  Long ago we had this huge discussion about how the T1/T2 arrows feel and how they arch too much (in my opinion).  After playing Arx Fatalis, I really liked how their arrows worked.  
They flew faster, went further and didn't arch so much, but did
arch.  So I thought Thief should have similar arrows and he ripped me to shreds about it.  Anyway, Thief 3 arrows are EXACTLY like Arx arrows  
I WIN!
 
WATER ARROWS: Aiming these are harder than I thought.  The crosshairs on the bow aren't set up quite right, they're a little off center, so it'll take some target practice till you can land an angle shot (say at a torch attached to a pillar).  
They seem to be a little touchy about the aiming but
if you get them anywhere near a torch it floods it.  Shooting guards holding torches with water arrows is good fun too... boy don't they get pissed.
 
GUARDS: A ton more variety.  Drunk, Fat, Old, Drunk fat AND Old, they're all there.  The comments they make now are actually BETTER Than T1/T2.  I actually laughed at something a guard said to me when he caught me.  Great stuff!
 
GUARDS II: They're tough, they notice a ton, they're incredibly smart, too smart for their own good.  They will often notice things have moved in a room you haven't even been in yet!  They'll instantly go on alert full alert if a nearby guard makes a comment and goes on partial alert.  They sometimes tell you where they're going though, which is cool.  "Hmm, think I'll go check behind that crate."  /me goes far away from that crate!  Sometimes in talking trash they sputter such a long contrived sentence it feels like a poorly acted B-Movie.
 
FLASHBOMBS: Flashbombs are different.  That's all I'll say in this post.
 
CUTSCENES: The Cutscenes are alright I guess.  I miss the old ones though
 
LOADING SCREENS: They show hints and weird quotes on the loading screens.  I think they're trying too hard personally.  The quotes are just not interesting like they were in T1/T2.  The hints are fairly obvious.
 
EXPERT DIFFICULTY: Yup, it's hard alright!  Guards notice EVERYTHING!
Civilians run like a bat out of hell if they suspect anything.  Loot goals are VERY difficult.  It's the only way to play though!
 
LOOT GLINT: This has it's advantages and it's disadvantages.  Basically
anything worth anything will twinkle slightly.  First disadvantage is that you can see loot from across the room screaming at you.  Kind of a dumbed down feature.  Secondly the twinkle only happens when it's not being highlighted.  So if you're in close quarters, you have to look away slightly to see if it's worth anything, or pick it up.
 
PICKUP/DROP: This is also different.  More on that in the other post.
 
Ok, I think that's about it for now.  I'll add more later if I think of
anything.  Sorry for the lack of order, I'm throwing down thoughts as they come to me.

Not too much to add to other's posts, but it's definitely Thief.

 
For the record, my system is 2000 Athlon XP, GeForce 3Ti 128mb and
512mb of system memory.
 
So far it runs perfectly smoothly, in 1024x768, with details and
shadow set to around 25%.  Bloom is turned off, as I hated that in
DE2.
 
I'm going to increase the settings now, to see how that affects
things.

Multiple sources tell us that all is not well at Deus Ex & Thief 3 developer ION Storm. Apparently some 20 - 25 people have been laid off at the Eidos owned Austin based developer. That's not all though as Warren Spector is said to be leaving the company as well. If we receive any more details we'll be sure to post them.