1. Martha Jones keeps getting cooler and cooler. And cooler! Let's face it, Rose wouldn't have come up with a plan when being chased by giant snapping crab claws. She would have shrieked a lot and waited for the Doctor to save her. Martha comes up with an idea, admits there's no proof it'll work but it's worth a shot, and hey presto, it buys them enough time for the Doctor to save them.
Martha, however, then expresses the same faith in the Doctor that Rose had, the same belief that viewers for over 40 years have come to hold as well: that the Doctor's always gonna come through in the end and fix it.
(So if and when he admits failure...)
2. Another point for Martha: She gets the Doctor to reveal the most closely-guarded parts of his past. Both Nine and Ten have dropped small, bitter hints about family and friends (isn't this the first mention of such since Susan, who was admittedly written as Hartnell's granddaughter to avoid any hint of innuendo?) but Martha got him to really start talking. Even moreso than Rose could. Okay, Rose could get him to open up a bit and discuss the Time War because she'd look all cute and concerned, and sure, you can get a bit ahead with that kind of attitude. But Martha sees through his verbal sidestepping, calls him on his evasiveness, pulls up a chair and says "Okay. You're going to give me the straight scoop, NOW." And, realizing he's been lying to himself as well as Martha (also still feeling guilty about lying to her and then almost getting her stranded on New New New New New New New Earth because of it) the Doctor starts to tell the truth. Very nice, and it didn't involve Martha using one of those "Honesty" patches as I had begun to fear once she started calling his bluffs. So hey, bonus cookies for avoiding that horrible device.
3. I replied to zhym with some of this, but I think one of the reasons RTD did a great job with Gridlock was because he didn't have to write an actual, specific, talking villain. The bad forces in this story involved the Bliss virus, an automated city support system, and de-evolved giant crabs. Thus, there was no opportunity for the typical strut-around-acting-all-evil-and-camp scene that RTD is quite fond of writing. One of his real weaknesses is his inability to write decent, original villains who do more than just strut around, acting all evil and camp.
Revisiting established villians, such as the Daleks and Cybermen, is one thing. But with the exception of the series ending two-parters, those characters have been left to other writers. I'll give RTD props for the Dalek-Cybermen trash talking, though.
But he seems to believe his own villain creations need to strut, they need to cackle, they need to roll their eyes and muahahaha, and they need to give themselves away with cutesy lines (Here, let me try: "Why hello, Doctor, so nice to eat -- er, I mean, MEET you. Heh, heh, heh.") Now the Love & Monsters villain was, granted, created through a children's write-in contest, but the concept was real good. RTD couldn't write him worth beans. And speaking of beans, the less said about the Slitheen, the better.
Here's a case in point: Anthony Head's character in School Reunion (written by Toby Whithouse) was able to bring more real menace, malice, and character to a single scene (the swimming pool confrontation) than the Slitheen were able to do in all their episodes. Why? Because his character didn't just act evil, he was evil. Compare that to Granny Plasmavore, who by all rights could have been written as a truly evil bloodsucker in benign disguise, rather than the old lady who cackles "Oooh, thank goodness I brought my straaaaaaw..."
Maybe RTD is intentionally lightening up his villains to avoid making them too Behind-The-Couch scary. Even so, other writers in the series have been able to write serious villains without going overboard on the fear -- but when fear is called for, they do a damn good job of it.
4. Loved the dystopia, loved the automated shutdown with no explanation to the drivers, loved the sudden drug dealers, loved the elderly couple (especially the carspotter!), loved the incredible scope of the stacked network of cars, loved the Face of Boe, loved hearing what we'd been hoping to hear all along. The bureaucratic stonewalling of the no-longer-existent motorway authorities was also very much something Douglas Adams would have loved. Loved to write about or contemplate, at least. He probably wouldn't have loved to have experienced it.
5. One more slight point of snark: My, it was interesting how people in this alternate future Earth were so kind and polite and forthcoming. "Hello, stranger we just kidnapped! My name is Milo, and this is my lovely wife Cheen. Let us tell you our whole life story!" -- then, no less than five minutes later -- "Hello, stranger we just rescued! My name is Thomas Kincaid Brannigan, and this is my lovely wife Valerie. Let us tell you our whole life story!"
It's a good thing the Doctor didn't have to endure introductions every time he dropped into another car ("Hello, stranger we just encountered! My name is Naked Guy, and this is my lovely Naked Wife...") or he'd never have made it as far as he got. Then again, Cat Nurse would've caught up with him much sooner if that had been the case...