I have tried to avoid spoilers. This certainly won't ruin the series for you.
STORY - 7
The plot itself begins as seemingly trivial and escalates gradually to reveal plans of world domination on the antagonists' parts. The series does well to incorporate elements of 20,000 Leagues Under the Sea with another overlying plot; that is, there's little originality to be found, yet I felt the plot was assembled well (though I enjoy sci-fi), with some intricacy and surprises to be had up to the end.
The one thing that struck me the wrong way was the uneven pacing. Some stretches of episodes are action-packed and progress the plot, pulling me onward through the series. Other episodes are pure filler that tend to drag on (and which I wanted to skip over). But overall, the story left me satisfied.
ANIMATION - 7
For when it was made ('89-'91), I thought the animation was great, for the most part. In plot-filler episodes, it's clear that standards were raised. There's plenty of detail to notice, from animated gauges and flowing hair to Gundam-esque explosions and frayed/melted metal. Action scenes were fluid to my eye (for twenty years ago), and character designs were varied and unique with a bit of an emphasis on realism instead of being overly stylistic.
Still, standards went out the window with the filler episodes. Concern for detail and perspective is somewhat lost, leaving characters looking flat and the anime coming off as childish.
SOUND - 6
Again, it's hit or miss. Sound effects were approriate and nothing seemed out of place, but nothing was particularly impressive. At times the background music was excellent and at other times terrible. Opening and ending themes were fine and seemed to fit the spirit of the series. Voice actors (Japanese) did well, I think. I sampled the English VAs and didn't care for them.
CHARACTERS - 6.5
The characters will make or break the series for you, I think. Initially, they seem to be caught up in their own shallow problems and fantasies, but their actions make more sense as the plot fleshes out. They each have their roles to play (well...most of them), and each makes some sort of developmental progress by the end of the series.
One problem is that a majority of the characters come from some sort of a ruined past, so you end up with a group of independent people with little other excitement in their lives to draw them away from the main problem in the story, which I find unrealistic and is my main complaint. My second issue is that, as the series struggles with poor humor, it is also plagued by sappy dialogue and a fickleness in the mindset of the characters that adds nothing to either the plot or the relationships between characters.
Don't be put off by Nadia's first few episodes. My inital impressions gave way to thoughts that the series as a whole would be shallow and childish. It is clear that the series catered to a younger audience and is plagued by laughless humor, but Nadia doesn't shy away from being more mature at points with some adult-friendly humor and onscreen blood and death among the many action sequences. Nadia won the Animage Grand Prix in 1990, putting it with the likes of Yu Yu Hakusho, Sailor Moon, and Gundam. At times you have to bear with it, but at others you'll be fully engaged.