Apologies for wall of text here...
I'm going to buck the trend a little bit and say Dishonored is an 8-ish out of 10. It's a phenomenal concept that is executed almost perfectly, but in my opinion it really, really, really loses impact toward the end. Some of the setpieces in and near the endgame are very clearly designed to increase the dramatic tension, but I personally found the low chaos ending-- which is, theoretically at least, the best one-- to be highly unsatisfying. It didn't really bring closure to the experience in a way that was emotionally cathartic. Without spoiling anything, I feel comfortable saying that the high chaos ending was much more... appropriate, I guess is the right word.
Basically, it felt like my whole first playthrough built up to a great final moment that just sort of petered out into nothingness. There was closure, yes; just not in the way that I feel would best suit the overall plot and theme of the game, and definitely not in the way that would suit Corvo as I played him.
Overall though, I really enjoyed the experience of playing it, and I'm sure I'll have at least another couple runs through the game. Some of the missions were among the most creative I've played in an FPS, and presented unique and interesting challenges. I always enjoy games that give a nonlethal option, and Dishonored definitely delivers in that regard. The gameplay is rock-solid, and all of the mechanics (with the possible exception of the contextual choke, which sometimes has issues from angled approaches) work really well.
So I guess I'll go with 8.5 out of 10. It's a great game that definitely deserves to be played by as many people as possible, but I feel like it falls a little bit short of its potential in a way that really undermined my overall experience.
I've also just finished Black Mesa, which I'm going to say was more or less the best FPS I've played in the last year. 10/10 easily. It basically takes every aspect of the original Half-Life that was good and makes it infinitely better while completely getting rid of, or restructuring, the less-positive elements. Pretty much everyone knows the plot of the first Half-Life, so there certainly weren't any developments in that department. However, the massive improvements in pacing, graphics, audio, level design, and even dialog really enhance the experience, and make the existing plot much more immersive.
Gameplay wise, there isn't really anything new to see here. If you've played Half-Life, Half-Life 2, or pretty much any other Source game, you know what you're getting into. The only complaint that I have is that if you aren't familiar with crouch-jumping, and the particular hand gymnastics it can sometimes entail, then you will not be able to beat the game. Period. It's not hard at all to get used to, but there are sections where a small miscontrol can easily result in an otherwise avoidable death.
The new soundtrack is absolutely incredible. Every setpiece track felt like it was designed just for me as I played through the game. Even the ambient pieces are solid, and for the most part non-repetitive. However, what really stands out for me about the soundtrack is that it's very listenable-- not just a select track or two, but the entire OST as a whole feels like a professionally produced album, and has a coherent quality that a lot of soundtracks just can't compete with. Unfortunately it's only available in mp3, although I *believe* that the original files are contained in the game data folders in .wav format.
Overall, as I said, one of the best FPS titles I've played in a very long time. It might not have the gameplay depth or graphical polish to compete with the likes of Deus Ex or Dishonored, but as a concise, cinematic gameplay experience it's very hard to beat.
Plus, being free is nice, too.