Many futuristic mecha-based anime survive on the creative and awe inspiring design of their mecha; however, Neon Genesis Evangelion is a different kind of anime. While the mecha are certainly original and cool looking, Eva does not rely on aesthetics alone.
What separates Eva from 95% of other mecha anime is its excellent story. Using the concept of god is not an entirely new ideayetEva takes religious symbolism and philosophy to new levels or depths, depending on your perspective.
But in order for the story to progress, the characters must be present.
Throughout the series, each character grows and matures into a wholly new person; and while it is not easy to identify with a Gendo or a Fuyutsuki, most of the characters draw some basic empathy from the viewers. By the same token, the characters would not have been what they are without excellent seiyuu. The voice crew never failed to bring out the soul of even the smallest side characters.
Most people believe that the fact that the series went over budget was what stopped Evangelion from being truly great; and, sadly, they are correct. Latter episodes featured excessive fanservice which was indubitably used to draw viewers' attention away from the massive flaws that were becoming more and more apparent in Eva, thanks to budget problems.
Gratuitous nudity began to appear in several episodes in the second half of the series. Though it was a huge contributor, the money issues are not the only thing that stoppedEvangelion from being truly epic.
Truly, the worst part of Evangelion was the ending.