Qualifications
Anyone can enjoy a great landscape, but few can create one. Good landscape designers need to know all about drafting, architecture, geology, and, of course, horticulture. Of these, horticulture is what is lacking most among the majority of even the best landscape designers.
It's uber-important to know the personalities of the plants that go into the landscape in order to know how they'll behave after the landscape is installed. Many of the best landscape designers actually got a degree in horticulture and got into landscape design, instead of getting their degree in landscape design and just trying to learn about horticulture as they went along (source).
There's no messing around in landscape design. (Only those who install the landscape get to be messy.) Unlike many of the other horticultural industries, which are overrun with folks who don't know or care about plants and would be just as happy working somewhere else, landscape design expects at least some degree of relevant formal education, and hopefully, experience.