I'm saddened to see no other responses in this topic. I shall post my own opinion than.
Enslaved, while it's no major innovator in the videogame field, does have quite a few charming qualities to it.
Just like you, TC, I game for Storyline first and foremost. I love a good storyline, good dialog, and beautiful cutscenes. Enslaved honestly did not have a very good storyline. The way all of the elements of the world around you fell together was beautiful, but it left numerous holes that I can't help but feel like I've been cheated. The ending was amazing, but I feel like there should be so much more past it that we never got to experience.
You have this mysterious entity known as Pyramid that is capturing "slaves" and maintains its status as an unseen threat throughout the entire game, but not once do you ever encounter this group through the game. Instead you're left fending off rogue mechs, which I assume were responsible for the destruction of civilization hundreds of years back.) Pyramid, who claims to be preserving humanity through memories, is apparently controlling mechs, but building them as well (as is the case of the Scorpions and Leviathan. He claims he doesn't slave humanity, but yet they're called slaves in the 1st chapter. I believe he's sincere in his explanation at the end, but there are contradictions in the storyline, which is a fault on the behalf of the developers.
On top of that, the revelation of what Pyramid is doesn't occur until the final chapter, and yet there are so many questions left unanswered. What was pyramid hoping to accomplish? Where are the slaves going to go now that they're freed? Is society going to attempt to rebuild now that they're gather in such large numbers? On top of that, no clarity was ever given to the discovery of the memories throughout the game except for what Pyramid tells us. No story was told. No bridge was made to link the different visions. Just random clips.
You play through the entire game with all of these drastic details lingering in the background, but instead all you do is spend half the game trying to escape New York, than the other half finding Pigsy and gathering parts. Only the very end held any semblance of a major storyline. The rest of it really just acted as plot filler.
The game does, however, shine in the plot, dialog, and character development. From the very beginning you can watch and see as Monkey and Trip begin to trust each other. They forced themselves to get alone in the beginning and you could see the rough edges of the relationship based on how they spoke to each other, but as the game progressed you could see how they more easily bantered with one another and wouldn't hesitate to do as the other asked. The things they had to say were humorous, but serious and helped develop the characters beyond who they started as in the beginning, something very few games have ever managed to do right.
Like I said before, the levels were mostly plot-filler and avoided the big picture, but they also stayed true to the Journey from the West. It showcased the adventures of an unlikely couple and the perils they endured as they grew clsoer to one another. And while I personally feel the developers should've had more detail put into the Pyramid element, their ultimate goal was seemingly to develop these characters directly. They were the ultimate focus and Ninja Theory succeeded with flying colors. The dialog, the enemies, the overgrown and war-torn city and even the command-based gameplay all aided in building Monkey and Trip into so much. For that, I give them an A+.
TL;DR: Overall storyline sucked, but minor details were excellent.
(If you're interested in a good storyline, I personally recommend Advent Rising, Breakdown, and Beyond Good & Evil. All are on original Xbox. AR doesn't work on 360, but Breakdown does and BG&E is getting an HD remake on XBLA soon.)
Fear not, my brothers! The next generation of gaming is upon us! Be glad! A reward for all your toil and all your sacrifices is upon us.
There are those who said this day would never come. What have they to say now?
(Yes, I ripped off a quote from Halo 2 and modified it to my own purposes. Sue me.)