You seem to have missed this peculiar characteristic of the 2006 definition of a dwarf planet: it is not a category of planets. Have a look here
The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.
So this definition actually enrages three kinds of people:
- People who think Pluto should be classified as a planet for historical reasons.
- People who think Pluto should be classified as a planet, precisely because as you said, they are many categories of planets which are quite different (terrestrials, gas giants...).
- People who think it is gramatically incorrect for "dwarf planets" not to be "planets".