It's an interesting progression. Halo 4 is much more richly-detailed and has a vastly more refined appearance, but a lot of compromization was involved.
For instance, Halo 4 creates more headroom for the quality of static stuff by minimizing the amount of vibrant gameplay-related dynamic effects. When it comes to stuff like explosions, Halo Reach uses a much heavier sprawl of particulars and transparent layers; and although Halo 3's full-resolution transparencies and dual backbuffer severely limit transparency fillrate, the game's investments do pay some dividends. For example, the extreme HDR depth allows the game to do
this by merely drawing a single texture layer:
Similarly, the quality of dynamic lighting was paired way back. Halo 4 uses huge numbers of small diffuse-only point lights (this is very cheap to do deferred), but this is just about
all it uses for gameplay-related effects. The game aggressively avoids using large lights or lights with specular reflections. It also avoids spotlights: the obvious example of this is the omission of a flashlight mechanism like existed in all prior Halo campaigns, but even things like vehicle headlights are actually rendered as point lights that float in front of a vehicle. This ends up looking flat-out bizarre when driving a tank on rough terrain in Halo 4, as you can watch the pointlights that make up the headlighting bobbing in and out of the ground.
This isn't just a compromise compared with Halo Reach or Halo 3, it's a compromise even if we're comparing it to Halo 1 on the original Xbox. Halo 1 has many large light sources, most prominent light sources cast specular reflections, and the game uses very large spotlights for the flashlight or vehicle headlights. Halo 1 is able to use large high-quality light sources to highlight important gameplay events (such as the location of a plasma grenade), and the specularity really helps to sell the game's lighting and materials:
Stuff like these sorts of lights, or the different use of transparencies, adds a lot of vibrancy and expressiveness in motion that Halo 4 sorely lacks.
There are a lot of other subtleties, like how Halo 3's ambitious lighting model allows it to use very complex mid-glossy materials without suffering from the hazy rim lighting that plagues Halo 4 (and that even plagues a lot of eighth-gen games). The Order 1886 is a notable example of a game which
doesn't have this issue... and it's a game where the developers directly referenced Halo 3's lighting system as an inspiration for their approach to indirect specular lighting.
A great example of fine-tuning visual trickery for the use case. Loads of areas in RSC2 look silly if you stop and look around, like how forests are often just a row or two of trees backed by a tree-textured wall.
At the end of the day though, I don't really have a strong visual preference for one game over the other. RSC1's clean and clear style has its own appeal for me, especially on the snow courses. They're certainly both
incredible games.
One thing where RSC1 has a big edge over RSC2 though... switching between 60fps gameplay and 30fps replays is bullshit.