This is how you do an episode of Doctor Who! It's just, wow!
1. Love the Douglas Adams references. "Junkyard at the end of the universe" is probably the more recognized one, as it references The Restaurant at the End of the Universe the second in the Hitchhiker's series. Personally, my favorite reference was the paraphrased quote from The Long Dark Tea-Time of the Soul. When Dirk Gently describes how zen driving works he explains that "I may not have gone where I intended to go, but I think I have ended up where I needed to be." Similarly, when the Doctor yells at the TARDIS (or Sexy) that she hasn't always taken him where he wanted to go, she responds that she's always taken him where he needed to be. I loved the subtle homages too Douglas Adams particularly since he worked as a script editor and writer for Doctor Who back in the days of Tom Baker.
2. Idris/TARDIS/Sexy. This character was played to perfection. Her nonlinear points of view made so much sense. Why should the TARDIS see things as a linear progression of cause and effect? Of course she'd see time as a big ball of wibbly-wobbly, timey-wimey stuff. (Sorry, had to reference another great DW episode there). "Biting's excellent. It's like kissing, only there's a winner.". That line made me laugh so hard. However, I loved the way she forced the Doctor and us to examine the significance of the TARDIS. Yes, for years the show creators have made her a character in her own right, but by actually letting her speak it pointed out just how much she's done. I loved the idea that it was she who stole the Doctor, rather than the other way around, because he was the only one mad enough. It gives new meaning to "mad man with a box". Also, I saw a bit of Jenny in her, especially the scene where she points out that the Doctor never follows directions (It reminds me a bit of the part where Jenny uses logic to prove he's a soldier). I also liked how she could have done this by referencing Amy's Choice when he reveals that he threw out the manual, but instead points out how he opens the doors.
3. Amy and Rory. Okay so yet another death for Rory. What's he up to know? I liked how they called back to how Rory waited for her in the past for 2000 years. I found it especially interesting because when they met up again then they were lovey dovey but here, where he presumably waited less than 2000 years he had written "Kill Amy" all over the walls. To be fair, he wasn't really dead, it was all House messing with Amy's mind. Which made me like it even more. For so many episodes we see Rory having doubts about Amy's love for him, yet here we see Amy scared that Rory could turn on her, that by making him wait he will stop loving her. I liked that they showed her having insecurities about his love too.
On that topic, seeing that Amy's mental definition of delight was her wedding to Rory was a nice reinforcement that she does truly love him.
4. The TARDIS graveyard. How amazing was that? I almost wish they had been able to go with Neil Gaiman's plan to have the chameleon circuits make them all look like broken washing machines, but it was still a beautiful scene. And then they just built a TARDIS (the winning design from that contest a while back).
5. Continuity. This episode was originally supposed to be in the previous season. Then it got moved. I think the glory of the stand alone episodes is that it didn't seem out of place. It's a fantastic story and a reminder that even with these huge overwhelming arcs, sometimes you need to go on a side adventure. Having it be that the reason they got sidetracked was the Doctor looking for another Time Lord gave it significance so it wasn't a random trip. Obviously, something Idris said plays into the mid season finale, but that's a line that even if it wasn't there, the episode would have been amazing. Similarly, had there been even more cryptic clues of what's to come, it would have also worked well. Since Sexy is the big wibbly wobbly ball we know she sees things out of order so we know it's not meant to make sense. So it doesn't matter if you watch the episode alone or as part of the season.
Finally, 6. Just some favorite quotes I haven't put in yet.
"She's a woman and she's the TARDIS."/"Did you wish really hard?"
"I wish I could do this for you. Actually, I don't. It's going to hurt a lot."
"Bunk beds are cool. It's a bed, with a ladder. You can't beat that." (Here's hoping that's true as I'll be having the top bunk at university).