Some random observations:
1) I am quite tall for the Iberian Peninsula (both Portugal and Spain). Headrests on buses and cars hit me in the back and I tower over pretty much all of the women and many of the men. I'm only very slightly taller than average (5'6" with average being 5'5" for women) in the USA, so it's weird feeling so very tall.
2) Madrid is a dog city. I noted that when I was here in 2007, too, and I love seeing all the dogs. I get to pet and play with a few particularly friendly ones, too, and that's grand. I do wish it was less of a dog poop city, though. Many people do pick up their dog's poop with little baggies and toss it away appropriately, but many also do not.
3) Travelers' diarrhea is really unpleasant. I kept getting it here the first few weeks and can't figure out what the precise culprit is. I feel that since I lived here once before (admittedly ten years ago) it is wholly unfair that it keeps happening (three separate occasions thus far).
4) Madrid is getting ever so slightly better with vegetarianism, but it's still very hard to be vegetarian here if you want to eat out.
5) Despite having lived here before and this being the case in other cities I've done residencies in as well, I'm still not entirely used to shops closing from 2-5pm. I like the European mindset toward work-life balance, but I'd prefer shift workers such that the stores could stay open.
6) If you live without A/C in constant 100-103*F weather, having one day that's overcast and merely 96*F feels markedly better.
7) Many Spaniards really don't speak English. I do speak enough Spanish to get along, but Fari doesn't speak any Spanish and I think she's surprised at how much it hinders her here - for a big European city like Madrid, the proportion who don't speak English is probably higher here than almost anywhere else of a similar size.
8) The flat I'm in has no microwave, no oven, and no pot with a lid. This severely hampers what I am able to cook. I'm also nervous that eating fresh fruit and vegetables is part of what's contributing to the traveler's diarrhea. As a result, I'm eating out a lot.
9) People drink non-alcoholic beer here surprisingly often. I typically only see it on offer in Muslim-run restaurants in the US. I only drink decaf coffee, so I get liking the taste of something but not the drug within it - but the cheap beer served everywhere here, Mahou, is to me not something I would prefer to other drinks without the alcohol...
10) There are more Asian immigrants here than ten years ago - a lot more. I used to walk around with an Asian friend in 2007 and people would scream "china" and run over to stare at her like she was in a zoo; nowadays there are "Chinese bazaars" on almost every street run by Asian immigrants.
11) There's a couple species of invasive small green parrot here. The more common one, the Argentinian parrot, has a very loud, annoying call. They're surprisingly hard to pin down in photos, but I've encountered them a few times.