Mild estery overnotes are okay, but not too much. No hop aroma should be present. In other words, they are supposed to be perfectly-balanced strong, dark ales. There are several porters available on the market, each with its own profile. The style has two substyles.
Robust porter is medium to full body and has a sharp note from chocolate or black malts.
Brown porter is lighter, with lower malt sweetness and no strong roast character. Commercial robust porters include Anchor Porter and Sierra Nevada Porter. Brown porters include Yuengling Porter and Samuel Smith's Taddy Porter. There is a lot of nonsense still being bandied about regarding the origin of the porter style. Furthermore, the estimable Mr. Harwood seems to have never even existed.