1) She is besties with Ten AND doesn’t want him romantically.
As we’ve already discussed, there’s nothing less appealing in a companion than an unrequited crush on the Doctor. Donna let’s us know from the very beginning that she isn’t interested in “mating” with her alien boy. The relationship they have is completely platonic and the Doctor is better for it. When he loses his best friend at the end of series 4, that’s when the Doctor becomes the creepy Timelord Victorious, and we all know what that leads to.
2) She’s the most awesome temp in Chiswick.
Shorthand, filing, and at least 100 words per minute. Donna’s secretarial skills not only get her a high profile secretary gig in London, but help the Doctor discern why the planets have disappeared in S04E12&13 and she figures out a pretty complicated numbering system of an alien race S04E06. Even though it hurts when the Doctor tells her she isn’t “special” when he first meets Donna, we know, by the end of their time together, he definitely feels differently.