One of John's cycling jackets says "Do not wash in biological detergents." Why?
The jacket is a soft polyester fleece that has been coated with a fluorcarbon chemical that gives it a durable water resistant (DWR) finish. As a result, it isn't truly waterproof, but it overwhelmingly repels water: water beads up and then runs off, thus doesn't soak into the fabric, keeping user dry. Chemically, biological detergents bind to the DWR and thereby interfere with the micro-function of it; letting raindrops soak straight thru.
Solution? Well... that's where things get complicated. Most DWR-coated garments imply that you can safely use an ordinary non-bio laundry detergent. Some manufacturers advise against even that though, and say that their products should only be hand-washed with delicate/specialist/"pure soap" detergents, and even then, maybe only rarely.
What else damages a DWR? Dirt itself interferes with the DWR-layer function. So you may have to wash frequently to keep water repellent properties at all. Over time, flexing and bending or smashing the fabric rubs the DWR off. UV light (sunlight) may also break it down. Fabric softener is a no-no with DWR, too, because it puts an extra coating on the DWR (again interferes with DWR function).
What's the BEST way to wash a DWR-coated jacket?
Many manufacturers recommend that you use a specialist detergent formulated specifically for DWR-coated items. This may be essential given that dirt itself interferes with the DWR, and that John's jacket (for instance) says "wash at 30"; because most non-Bio detergents aren't formulated to be effective below 40 deg. C. The specialist detergents are expensive, though, and not easy to get hold of (you'll probably have to order online).
How do I fix the DWR on my waterproof? First try washing the item in non-bio detergent, many users report good results with this simple step to remove bio-detergent residues. If still not satisfied, there are several products that aim to restore the DWR layer: Nikwax and Grangers are the most commonly-mentioned brands. These typically require ironing or tumble drying as the final step in the reproofing process; although some restoration products work within the wash itself. You may well have to research and experiment to find which procedure/chemical will work best for you.
What about Goretex and biological detergents?
Goretex isn't just a coating, it's an actual fabric (sandwiched between durable cotton layers, typically). Some care instructions also imply that Goretex items shouldn't be washed with conventional detergents. Fabric conditioner is always out with Goretex.
Does it really matter that much?
Some report that they have for years washed in bio detergent their DWR-coated clothes -- without any noticeable loss of performance. These may be less demanding users, though. And given how expensive high quality DWR-coated items are, you probably don't really want to be running the experiment yourself.
What about cheap waterproofs, the ones that feel like plastic? PVC coated. You'll know if you have this kind of (true, genuine) waterproof. These will also last best if treated like DWR-coated garments. However, the PVC layer is often thick enough that PVC-coated items are relatively much more robust and you are more likely to get away without special washing care unless washed relatively frequently.