As I have mentioned before my most frustrating weed in my yard is Caltrop. Although it has to be admired for its persistence and ability to grow anywhere, and at a rate of knots, and the amount of seeds one plant can put out, which I swear most end up growing, but not for its ability to find bare feet!!
Moving into a different area to what I have grown up in came a whole lot of new weeds. My attitude was if I didnt know what it was it was allowed to stay, then once it grew if it didnt have prickles it was allowed to stay a bit longer. Then as they have flowered I have set about identifying them. Many of the plants that I had never seen before ended up being natives, most considered as weeds because they are so common but native never the less.
The previous owners used to religiously spray the whole backyard to get rid of the "weeds" so when we moved in it was completely barren, since I have moved in I have not sprayed at all (although I have been tempted by the caltrop) and have a range of "native weeds" that have sprung up. When something grows which I havent seen before,I take either a photo or a sample of it into work to try and have it identified by our environmental person, thinking I may have just found a rare and endangered species (well I have never seen it before). Most if the time I have been told that it is a native but it grows like a weed, grows like mad, producing lots of seeds (which will stick in your socks, but not really prickly like caltrop), then die off, but the beaded dragons and sleepy lizards have been seen to eat it, So it earns the right to stay in the yard.
I have also had a few acacias that have grown as well, (I am assuming out of bird droppings), these will definitely be staying, if these want to grow by themselves it saves me trying to buy tubestock and keep them watered all the time. As well as the fact that they have decided to grow near the side fence which I want to create a native screen against, to create a bit of privacy and also provide habitat for birds and lizards. Usually it is the self seeded ones which grow the best.