I've reached Trevor now and just entering into the Grand Senora for the first time... some of my initial impressions from LS...
-Best part of the game is the new traffic. Variety must be 5-6x time better... even with 20-30 cars on screen there are not many repeats. Not only is there more variety but the sample pool is much bigger. All the DLC cars spawning help a lot. There are even new 'modded' cars like baby blue Enus (Bentley) SUVs and bright green or purple Dewbauchees (Aston Martin) with spoilers, etc. It is such a huge improvement over the PS3 version, and it'll make a huge affect on free roam gameplay over time. So many more cars to steal.
-New foliage and weather (rain, wind, fog) is really nice. New lighting, maybe because I've played a lot of Driveclub recently, is nice but didn't especially awe strike me. The new foliage did though. Game looks gorgeous but still has a lot jagged textures, a lot of shimmering, etc. It definitely still feels like a PS3 game upgraded to PS4 than a natural PS4 game. Which is fine, expected, etc. Just saying the game hasn't really made me stop and stare at the new technical graphics... it's usually more about the art or sheer variety/quantity of art on screen at once.
-The new blood and gun models, the new explosions are awesome. Someone accidentally crashed into a gas station in LS and it was so huge, so loud. Scared the eff out of me because I was just hanging out with Lamar and it was so unexpected. The new blood, especially in cut-scenes when you get shot up, looks really good.
-The new music is very fitting. Radio Los Santos additions fit Franklin so well... I laughed so loud when I heard "I woke up in a new Buggati (Adder)". Same with some of the new Rock LS tracks for Michael. Rockstar has by far the best soundtrack team in the industry... they always pick the best songs, no matter how popular or obscure the track.
-Lots of small new cut-scene changes. I think they made Heist 1 start about 1 hour earlier than before so that the final scene has a stronger sunset. The first Franklin mission has a new song (replaced NWA). M has more blood on his face in one scene; other scenes have new cars, world objects, pets, etc. You can tell they went through every cut-scene and added new details by hand.
By far the best part is the new traffic variety, foliage, and weather, and those alone will make my daily free roam (since GTAV/GTAO is basically the only game I play) much better.