At the fundamental level, design is an art form and as such it can be highly subjective. However, there are several criteria by which design may be evaluated. A few that might be helpful include:

relevance.

Does the style connect with your target audience? Are the visuals relevant to their interests?

design integrity and consistency.

If you are developing an entire marketing campaign or a simple four page brochure, is the same look and feel carried out through the entire piece?

readability.

fonts too small? If your audience has a difficult time reading the piece they will often loose interest quickly.

Composition.

In a printed piece consider page layout and organization. Are the visuals and content arranged in a logical quick to understand sequence? Is your site structured so that users can find what they are looking for quickly and effectively? Are there are certain sections of your site you would like to drive visitors towards? Your pages should be designed accordingly.

Each brand is unique and design should always be evaluated in light of it's context, but hopefully these ideas will prove helpful as a place to start.