Ok, where to start.
Challenge. Or Dishonored's lack thereof.
From level design to puzzles to combat to stealth. it's all too easy, even on Very Hard with the majority of the hand holding disabled.
Level Design. Whilst Dishonored's level design is certainly above average, there is not enough variety or challenge (again). This is mostly down to the awesome Blink power, however (I have a love-hate opinion on bllink, more on that later). Level design is probably Dishonored's strongest aspect, but could have been better.
Gameplay.
Puzzles, Or Dishonored's lack therof. There are approximately three puzzles in the game in the form of a safe code being required and the player having to figure it out. There was only one I was just about satisfied with, that being the one with the numbers "hidden" in the paintings.
Lets take another one as an example: A code is required to open a safe. This code can be found by reading the conveniently placed book 1 foot away and 'deciphering' the contents inside. This is intentionally easy. I understand why (sales) but believe this is the wrong way to go about it. These puzzles were optional, so they can be challenging without any harm done for the more casual player. Furthermore, there are difficulty modes where practically anything can be coded to be exclusive to that mode. Developers should make use of this fact.
Finally, there were very few of these puzzles at all, whilst Arx Fatalis or Deus Ex are filled with them. Now I predict your next thought: "Dishonored is not Arx or DX", but when a game is marketed as an immersive sim then depth and intellectual challenge is what I expect (one reason I am not much a fan of Thief by the way
).
There are books and such everywhere, but no incentive to read them besides lore, unlike those aforementioned games.
This game has all the hallmarks of the type of immersive sim I love (Open level design, RPG elements, puzzles, books and such everywhere, option for stealth and combat) but it's all watered down to a Bioshock level (OK, not that drastic), which leads me to playstyles:
Combat:
This is just mindless. Blink around everywhere slaughtering guards and shooting em up. Playing the combat playstyles of other immersive sims is very rewarding, not like this.
Combat had little depth to it also, most enemies died in two-three slashes on very hard, most attacks you could counter with a instakill attack that had a huge time window and so on.
Stealth:
Whilst this is probably the "True" way to play Dishonored, people often complain that the stealth experience lacks the depth of older im sims. Going off of the combat, and blink specifically, I'm not surprised. I did do a fair bit of sneaking as the combat was boring me, but blink made sneaking boring too.
Blink:
This mechanic, well. Specifically designed to be abused since the mana bar always rechages to one blink's worth, this mechanic single handedly ruins any combat, stealth and platforming challenge Dishonored could have offered. However, I am torn. It's just so darn fun!
Blink's consequences on platforming: Tough platforming and traversing of a level? Not in Dishonored. Platforming IS blink, mastering it's use, which is easy.
Blink's consequences combat: Teleport behind people then smash 'em on the back of the head, Low on health or surrounded? No worries, you're gone in a blink of an eye.
Blink's consequences on stealth: No tense sneaking here. Teleport above one's head and plunge that blade into them. Then, again, teleport away in a blink of an eye.
Stop Time: Does this need explaining? Whilst there is a slow time spell in Arx Fatalis, it is aquired late in the game (if you find it in the optional masterpiece of level design, the crypt), is one of the tougher spells to cast, doesn't stop but slows time and drains mana real quick.
In Dishonored you can unlock this early and win the game, especially with all the mana potions about.
See through walls: No. This common power should be banned from games unless it's range is very limited and consequences/drain of resources are great.
Bone charms: Randomised locations to increase replayability. This is the wrong way to create replayability, in my opinion. You create replayability with RPG elements such as these by making the player unable to have/equip them all in one playthrough and by making the majority of them hard to find. The former they did do, granted, but the latter they gave you a heart which pin pointed the way. Furthermore they couldn't design tougher locations or quests to overcome when the bonecharm recieved could just be "eat a rat for a small mana boost" .
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RPG elements in general: See bone charms. Weapon modding. Powers. This is all just lacking (intentionally so, again) and imbalanced. Bioshock's level.
Inventory:
A radial menu. The bane of modern inventory/weapon select design. Immersion killing, stops time, not fluid. There is no excuse for the existence of this mechanic in any game.
Of course, I wanted a grid-based inventory too and the deeper interaction they provide 
The Heart: The heart, whilst optional, was always there tempting you in the radial menu. The heart & blink combined completely kills exploration, exploration having always been a primary selling point of immersive sims, for me. I think you could disable the heart in the options though, hazy memory here.
NPC's and writing: The writing above average for a video game from what I remember but weak for an immersive sim. However I didn't read all the books as I was not enjoying the experience all that much. Perhaps I am not worthy to judge here, I'm no writer, but it is widely recognised that Dishonored's main narrative is passable.
NPC's: not that memorable. Some stand-out characters, but just a few.
One point in the game I remember being sorely disappointed in was at the masked banquet/party. The NPC's had little to say, didn't react to stealing, just lifeless. Moving mannequins perhaps a fitting description of them.
Immersive Simulation Design: If you turn off all the hand holding then it's pretty strong, but where are the lengthy inventory item descriptions?
Interaction with the environment, whilst satisfactory, is not on DX or Arx's level. In Arx I any two items I logically think I can combine, I can. Combine a sword with a stick to create a sharp wooden spike, wood with rope for a makeshift fishing rod and so on. Anything you expect to be interactive, it is. It's great.
Now I get that they wanted Dishonored to be fast paced, but this could have been achieved without streamling to this extent.
Either offer a fast paced sneaker or an in-depth magnificent immersive sim. This is just a weak jack of all design aspects, master of none.
Length of the game: Too short.
I get that half of these complaints are more due to my expectations of Dishonored than what the devs intended it to be, but the other half is regarding what I believe to be flawed design. Still a good game though. 6.5/10 on the immersive sim-exclusive scale, 8 compared to everything else.
All this is without whatever the Thief fans have to say about it. Probably few & minor complaints knowing those Taffers. 