Yeah, Catherine Tate is fine in other roles, but Donna was terrible--always needy, never serving a true, independent purpose other than convenient writer shoehorning to make her "the most important" woman in the universe, when her original character profile was just wanting to get away from her boring life.
In that case, on the plus side, Donna rarely got any episodes where she got to be the sole companion. Of the 13 episodes in Season 4, she really only got 4 of them to herself ("Partners in Crime," "The Fires of Pompeii," "Planet of the Ood," "The Unicorn & the Wasp"). The rest of the time, she had to share screentime with someone else, either Martha ("The Sontaran Strategem," "The Poison Sky," "The Doctor's Daughter"), Rose ("Turn Left"), River ("Silence in the Library," "Forest of the Dead"), or everybody ("The Stolen Earth," "Journey's End"). The only other time when there wasn't another companion competing for attention was in "Midnight," where Donna only makes brief token appearances while the bulk of the story is about the Doctor trapped on an alien tour bus with a bunch of strangers.
Yeah...ugh. The constant chasing after the Doctor's affections made her appearances during the Tennant period as pleasant an experience as root canal. ..and the way the showrunners seemed to think she would be endearing after the way she disregarded / trampled all over Mickey's relationship with her, so she could explore her "better option"/fan-girling? That made her one of the worst companions.
Out of all of the characters during the Eccleston/Tennant years, Mickey was the one that I most related to. So it bothered me that Rose & the Doctor treated him so badly. When Rose started to get a bit jealous about the Doctor's feelings for Madame duPompadour in "The Girl in the Fireplace," I just thought, "Serves you right!"
I think that's part of the reason why I prefer Matt Smith's Doctor. When he saw that Amy was going to cheat on her fiancé on the night before her wedding in order to indulge her childhood crush on the Doctor, he immediately started bringing Rory along in order to head off a repeat of the whole Rose/Mickey situation.
Alternate history should not be classified as science fiction.
Depends on how the history is alternate. Is it just exploring a different but plausible sequence of events? Or is there time travel involved or something? IIRC, one of the big alternate history series involves time traveling Neo-Nazis going back and giving AK-47s to the Confederacy. There's another set during the 1940s where both the Allies & Axis powers have to put aside their differences and make an alliance to repel an alien invasion. (It had a really cool cover where Churchill, Hitler, et al were gathered around a conference table strategizing together.

)
I agree. Has there been any attempt to name that type of genre? What about something like The Handmaid's Tail? Definitely not science fiction, and it could be called "alternate history" but it's more like "alternate present." I'm curious as to how people classify stuff like that.
I think that the technical term is "speculative fiction." It's a broad umbrella that covers any story that depicts a world different from our own. "Science fiction" is a subset of that, depicting a world changed by scientific advances that we don't have yet. But when it's less technological and more about alternate social developments, then it's just speculative fiction, not sci-fi. Some examples include
V for Vendetta, The Book of Eli, and the
Mad Max series. (Be honest, we have enough weapons that we could totally desolate the planet to
Mad Max levels if we wanted to.)
The Handmaid’s Tale takes place in the “Immediate future”. The day after tomorrow.
Next Sunday A.D.?

(Those handmaids got off easy. At least they weren't kidnapped by some mad scientists, sent into space, and forced to watch bad movies.)