Hands down, the length of the game. The pacing through the story is too fast (your first REAL mission is essentially a chapter boss mission). Relying on the gamer being a completionist who will hunt down every rune and bone to make the game longer is NOT an effective means of adding gameplay value. I completed the game, with side quests I found out about, in ~10 hours. I was a little surprised when other people posted pretty much the same play-through times, and was edging on disbelief, or at least maybe being a little critical of their play style. BUT, I did NOT run through the game at all. I played Corvo as angry and vengeful, but not stupid. Thus, I stealthed through a lot of the missions, but did not hesitate to kill any who got in my way. After the first climix, I became incensed, and took out every last person in my way to the ending. Still, I approached with stealth and guile. Ten hours. That's it. That is, for me, less than half a game.
BESIDES the fact that "bang for your buck" stinks ($6/hour? Ouch.), replay value is not very good at all. Same missions, same character, same handful of powers. But it's worse than that. The entire premise of the game is "play as you will", with a hearty encouragment towards playing a good guy who stealths through with minimal to no kills (based on the endings). Yet, the pacing of the story/levels/missions is one that encourages FPS play. It's fast and frenetic, with too much happening too quickly. IOW, the pacing doesn't fit the game at all and would be more suited to a game that was designed with MP in mind (which the levels do seem to be), with a relying on MP to be the focus. Why they did this, I have no idea. Many of the areas/maps are "re-usable", and, indeed, are recycled for future missions. So the only reasoning I can think of is that they ran out of story ideas. My ultimate feeling is that I purchased a paperback novel with 300pp only to discover that the last 200pp are blank.
So, overall, I can't really give Dishonored better than a 66%, D-. Don't get me wrong, EVERYTHING else about the game is amazing. BUT, this one flaw is a VERY major flaw. So much so that, despite the greatness of all other aspects of the game, I would never recommend it to anyone other than a hard-core action/adventure buff.
Simply adding in-between missions (doubling mission count), lead-ins to "chapter boss" missions, and replacing some runes (bone charms are fine on side quests) to be more easily discoverable without having to go off in the exact opoosite direction of your mission intent would, IMO, fix this game up to near 100%. As it is, though, it will fall through the cracks as just another adventure game, CnP of every past adventure game. This is a shame, because otherwise, Dishonered IS something really special.
No replay value? What the hell are you smoking
No the game does not rely on people huntin down runes and bone charms for length as the game gives you an item to speed up that process. The length comes from the missions and the side missions which based on your time you ignored most of.
As for replay being same missions yes but you can play them completely differently. Go in a different way, take a different route through the area, use your powers differently, stealth vs aggressive, lethal vs non lethal. mix of the four.
Regarding same batch of powers try leveling different abilities. Ever consider that. The game doesn't have enough runes for everything so it is impossible for you to be incapable of using a different character build on a second or even third run. Or even go for the mostly blood and steel achievement and use only blink. This in itself adds more replayability. First run you blinked over obstacles, on your next try using possessions to sneak around, or none of the above and find ways through without using blink. There are plenty of different ways to handle every situation. Not to mention the changes to the levels based on factors like sidequests you have done and chaos level.
As for reused maps there was one section of city that was reused. Count it 1. And it was only used for the 2nd and 3rd missions (and only a portion of those missions at that not the entire mission.
As for being fast and frentic that is only if you made it so. On my second playthrough it was fast and frentic because I chose to go in aggressively. My first it was mid range because I was sneaking as much as possible while getting caught sporadically occaisionally speeding it up. If you play full on ghost non-lethal then it won't be frentic at all.
If you are only getting and average of $6 an hour then it is your own fault. I have made two runs so far together totalling around 25-30 hours during neither of which did I search out all the runes or bone charms. And within those two runs I haven't even touched the low chaos side of the spectrum. I am easily looking at around 2 more runs (non lethal low chaos, and lethal low chaos) which based on my stats so far will easily put me in the range of 50-60 hours of content not including any future runs I decide to do when I occaisionally come back to the game.