Scale and size (and a bit of lighting). PR1 did an incredible job at presenting the Kaijus and Jaegers as immense things. The lighting also helped to hide the CGI, and it makes them look a bit more real compared to PR2.

The way they move and interact with their environments also has a huge impact as well. Using container ships as baseball bats instantly shows how huge Gypsy Danger is, and having one of the major fights of the movie take place between skyscrapers makes them feel just as huge because they're making skyscrapers look tiny!

PR2 does very little or even none of this. The climax of the movie is on a mountain of all places. Mountains are huge, even compared to them. You lose their size amongst the backdrop and can't grasp the scale of them.

And lastly, the PR1 Jaegers are heavy fuckers. They're slow because they're gigantic. The PR2 Jaegers are a half-way point between Power Rangers and Bionicle.