Frame Level

The high budget of the To Heart TV series shows in its character designs, art direction, color design, backgrounds and illustration quality. You'd be hard-pressed to find a bad frame in the bunch.

As for animation that's been released over here...I suggest Ghost in the Shell. With its intricate backgrounds and gorgeous cels, every frame offers tons of beauty and visual information. Another one to study is Kiki's Delivery Service, where every detail tells a story and draws you further into Kiki's world.

Strong Poses and Facial Expressions
Just because you can't afford 24fps character animation doesn't mean you have to abandon the principles of good character animation altogether. Even in the held cels, make sure the character conveys emotions through body posture and facial expression as much as possible. At the very least, the character should have a natural posture and a clear facial expression. The next time you watch your favorite anime, study the facial expressions of the characters.

Illustration Quality
With held cels, the audience will have plenty of time to study the drawing. If the held cel looks cheap, the whole animation will feel cheap. For celshaders, this translates into making your 3D model as flawless and good-looking as possible. It also means making the pose, angle of the camera, and overall composition of the shot as strong as possible.

Angelic auras, demonic eyes, city lights, candles, flashlights, flourescent lighting, fire, lasers, explosions, headlights. I've seen glow used on all kinds of things in anime -- it's a little touch that can enhance the look of a shot. The next time you watch Escaflowne, study its use of both normal, edge, specular and diffuse glow. There it got used on everything from visible light sources to the bright edge of a sword. Fatal Fury -- the Motion Picture also uses some gorgeous glow and soft-filter effects -- even to the point of applying glow to a character's blood in an intense fight.