There  are certain things I will pay attention to when skimming through nice web pages.

In terms of design

Color

What’s the color palette like. Is it appealing?

Fonts

Do the fonts chosen convey the overall style of the design? In particular, has the western characters been treated separately from the multi-byte ones?

Layout (Information Architecture)

Is the overall structure of the page clear to the viewer? Are the elements organized in a predictable way, and are the important ones highlighted with visual treatment or grouping?

Metaphors (Visual Cues)

What metaphors have been used in the page to give the page a 3D feel, instead of a 2D print on a flat screen.

Usefulness (Readability)

Is the content of the page valuable and necessary? How clear and interesting the messages are?

In terms of implementation (limited to front end – html, css, and js)

Efficiency

Whether things are done in the fewest possible lines of code? Whether the developer is aware of the performance implications of  certain js features?

Portability

Whether the code works across browsers?

Maintainability

How hard is it to change certain things? Are the css organized in an easy to change or substitute way?

Fidelity

How much attention to detail has being given to fully replicate the original design? (This is often only valid when have accessible to design compositions).