So far the main thing I've learned is that prior to this I knew nothing about carpentry. I've made some little things in the past, but it was always just the totally uneducated "hey I'll stick some wood together and nail and screw it semi-randomly".
In reality the way modern American carpentry works is very systematic; the 2x4 stud and the 1x2 furring strip are not like multi-purpose legos that you can just fit together however you want; you use them in specific ways (and in fact the "stud" is not just a piece of wood, it is specifically engineered to be very strong in compression to bear the loads of stick framing, and not necessarily strong in other ways). You use specific sizes of nails for each specific task.
Some notes :
Garden beds are four 4x4 cedar with through bolts. I'm pretty happy with them in general but if I were to do it again I would not do the "high raised bed" style and instead just use two 4x4 posts (which gives you about a 7" raised bed, since a 4x4 is actually 3.5" , and you could use standard 8" carriage bolts in that case, I had to mail-order 15" bolts (actually better than carriage bolts are "timber bolts" which are just carriage bolts with special cutting flanges that hold the bolt better in softwoods like cedar)). Be careful when tightening bolts on wood, you do not want them to be really tight, just sort of lightly hand tight. To make the nut stick you may need two nuts that you tighten against each other.
Kitten loves the cat tower, and the fibrousy wallpaper stuff on it was pretty successful (she loves to climb straight up it, rather than jumping between the platforms usually), but it is breaking down; I'm not sure if there is any material that would be more resilient, perhaps some kind of woven bamboo mat? She can knock it over with her crazy stripper pole moves, we currently have it stabilized by leaning in a corner of the room; to really make it self-stable would require a really wide base or a lot of weight at the base, I think.
Potting table is standard apron-style table; it's 8 feet long so I used 2x6 apron rails. I pocket-holed the table top boards together, which I would not do again. If I made another table top I would probably just try to glue it (I don't have the clamps for that kind of glue-up at the moment); with an apron-style table the top is not really load-bearing anyway, the apron is the crucial structure, so the strength of the top joinery is not important. One thing you might not think of is the attachment of the top to the apron needs the ability to move side to side (but not up and down), so drill the screw shank holes oversize. (apron-style tables are super duper easy to make)
I only made the vertical trellis, the top part was already there, but with no way for a vine to reach it. One of my house pet peeves is when people have a trellis with nothing on it, so I had to do something about that. (one of the more retarded fads in house style was the "trellis top" fences that everyone was getting 5 years; hardly any of them actually have anything growing on them so there're all these stupid looking empty trelli in the fancy neighborhoods of Seattle). The only tricky thing about that trellis was trying to do the anchors to the house correctly. Piercing the skin of a house is almost always a bad idea and has to be done with care to avoid creating a water-incursion point. I used through-bolts to a block inside the sheathing. The bolts are angled down to reduce capillary action, and hopefully the caulk does the rest; the holes for the bolts must be over-size to avoid cracking the siding with seasonal flex. One mistake I made was running the bolts from the outside to the inside; I should have done it the opposite way, which would have let me install the block and bolts, and caulk and paint that assembly and let it fully dry before attaching the trellis.
Chicken coop - urr various things I would probably do differently here. It's over-engineered in some ways (more structurally sound than necessary for the weight of chickens), but also missing some important features, like a heat lamp (addendum : heat lamp was easily added via extention cord) and some easier access points. It currently has an egg retrieval door, a chicken ramp door, and a removeable wall and roof for cleaning access, but I think I have to add another door for human play time access. The run is also small compared to the coop, I may have to make a bigger run some day. Maybe the main annoyance at the moment is you have to get inside to open and close the chicken ramp door; when I made it I thought we would basically just leave that door open all the time, but we've been closing it to help them stay warm at night, and it's a pain to get in to open and close it.