My girlfriend likes The Christmas Invasion for some reason, but I can't stand it. First truly dreadful episode of the new show, and a harbinger of the downward plunge it would later often exhibit.
I consider the first series with Christopher Eccleston to be practically perfect, despite every episode being set on or near earth. But that unfortunate trend continues later--it's very bizarre that in a show with a plot device that literally go anywhere, anywhen, they keep coming back here, particularly in the present day. What the heck is that about? Use your imagination, writers, and show us some real alien worlds and strange things we've never heard of! Present day earth gets boring, and that's not what I want to see in scifi, even if they seem to think it's what people want. From what I've seen of the classic series, that was something they really excelled at back then, despite the limited budget and slower pace, and it's disappointing that the new show often fails to live up to it in that respect.
Just look at something like the Key to Time arc with Tom Baker and you'll see what I'm referring to. I mean, a planet that quite literally pirates entire other planets, complete with pirate captain who has a robotic parrot? A companion who is a fellow Time Lord and every bit as clever as the Doctor (if less experienced in travelling), and whose bickering yet respectful interaction with him is utterly fascinating to watch? Assembling an artefact with the power to stop time throughout the universe, using it to prevent two groups from destroying each other, only to break it apart again when it turns out the higher being who asked them to retrieve it is actually going to use it for evil? Now that's the kind of science fiction I can get behind!
Don't even get me started on the onslaught of emo angst that plagued the later Russell T Davies years. That kind of thing has its full expression on Torchwood, which I shun since it is a pile of utter garbage, but it interferes a lot in the enjoyment of Doctor Who as well. By the opposite note, the most recent series takes the complete opposite approach, having nearly no genuine emotional resonance whatsoever. Steven Moffat excels at creating complex and interesting plots and time twists and generally far more interesting ideas than anyone else writing on the show these days, but it has to be said that he often fails rather wretchedly at writing believable female characters. River Song's grating smugness succeeds only in being a massive annoyance, and Amy is a flat character devoid of any depth. (Hugely unfortunate since I love redheads and she is undeniably very pretty!) I know a fair amount of people didn't like Rose or her relationship with the Doctor, but she had heart and was infinitely more interesting than anyone else who has travelled with him these days.
It might seem, reading this post, that I don't actually like the new series at all, but that isn't true, because I'm quite a fan of it. (The classic series surely has its flaws as well, lest you think I'm saying that it was so much better or something.) But it's frustrating liking something that at the same time can be so flawed from the writing perspective, especially when the flaws are so obvious and could be avoided with a bit of effort.