Qualities for a good technical writer

1. Good..... command of language of course, like any good writer. Not the gift for flowery, high-sounding kind of language, but the ability to write clearly and concisely using simple words.
2. Good communication skills. A good TW should be able to tell things in a way that the listener (reader) can understand easily. Technical writing handles much data that has to come from subject experts and existing literature. The TW has to learn these from experts through discussions and interviews. This implies, among many other things,
3. Good common sense, a curious mind and readiness to learn new things
4. Knowledge of the subject area in which he/she has to operate. A TW in software industry should know the fundamentals of software engineering; good grounding in hardware also is needed since these are complementary. Technical writer in pharma industry can work well only if one has sufficient knowledge of chemistry, pharmacology, medical science and related areas. In short being knowledge savvy and tech savvy will help.
5. Familiarity with popular word processing and designing software used by Technical Writers.

That is not all. But I hope this gives big clues as to how one can evolve into a good technical writer.