Is there a real difference between Manga, Hentai, and Eechi styles?
Manga would be the comic-book/graphic novels, Hentai is the anime version of Hardcore Porn, Ecchi would be softcore (usually done for humor purposes)- think Porky's or that sort of thing.
What about draw styles. Bigger eyes, More Angular or thinner faces.
The art design elements of an anime character? That's pretty much dependent on a few things...
1. Who is doing the character design (for example, the person who did Gundam Seed's character design tends to keep to a similar look- as evidenced by his other work on Fafner, Scryed, and Gin-Iro no Olynsiss)
2. How close the character design sticks to the source material (be it manga or video/computer dating-sim games or even the original anime series in the case of remakes of an anime)
3. The time in which the anime was made (anime made in the 80's have a different look compared to those made in the 90's). One example would be to compare the original Macross/Robotech series to their later counterparts (Macross Frontier or Macross Zero for Macross and Shadow Chronicles for Robotech) or even the original Ghost in the Shell movie with it's later series (Stand Alone Complex, etc.).