Yes, there are so many things that determine the general "quality" of an asset.
The artistic side is a bit hard to quantify, especially with characters - how strong are the silhouettes, the proportions, is the anatomy correct, are there some characteristic features, are the colors nicely balanced etc. Valve actually has some pretty good guides available for DOTA that you can search for, although that game obviously has a very different art style and some specific requirements because of the camera angle and the gameplay.
But all in all 343's stuff is - IMHO - a marked improvement over previous Halo games, nicely balancing the added realism with the art style of the previous games. The Covenant still have colored armor with insect carapace like material attributes, the plasma effects are still quite saturated, the human weapons and vehicles and ships are still more practical and dull, and yet it all looks a lot more like real life, too.
The tech side is harder to guess, but the detail is certainly there, although the improvements are probably not as big as it was from Halo 3 to Reach. On limited hardware like consoles, it mostly comes down to efficiency. How much of the texture space is utilized is the start, but it's a lot about being good with optimization - mirroring a lot of the texture without it being obvious, removing triangles that don't contribute to the silhouette, using mask textures and vertex colors and packing multiple maps into the RGB channels of a single texture and so on.
For example I can tell you that the stuff in Mass Effect is absolutely top of the line as I've worked with Bioware's game assets. I can only guess that Naughty Dog is pretty good at this too; and it's logical to assume that 343's artists know their craft very well.
So it's probably a case of building better looking, better designed stuff with better efficiency. But keep in mind that Bungie was only to their second generation game with Reach, and 343's crew is now probably mostly on their third. So they're more experienced, and I'd expect Bungie's new game to look better than Reach, too.
Not to mention that Microsoft has probably payed a lot of money to recruit top quality talent. Like art director Kenneth Scott and concept artist Nicolas Bouvier (Sparth) were 'stolen' from id software well before Rage was completed. I mean just go look at
Sparth's stuff, it's absolutely incredible. Or the audio, they hired Kojima's guy and the producer behind Massive Attack...
As for the tech, Bungie released a lot of papers at GDC from all their engines. It's how we know how the HDR in Halo 3 was resource intensive, or how there's an advanced impostor system in Reach. Once H4 is done and released, I expect 343 to follow this tradition as well.
