Awesome thread! Thanks for providing PLENTY of food for thought.
BTW, since there's so much info here, one thing I didn't see mentioned are Sawyer filters. Backpacking filters that are designed for gravity fed setups and even potentially direct feed setups (attach filter to squeeze pouch, squeeze the pouch and filter water directly into your mouth).
The filters use an interesting technological innovation used in dialysis machines. Basically, hundreds of microtubes in a semi loop. The microtubes have tiny holes along their lengths. The manufacturing process produces consistent hole sizes and water enters the tubes through the holes and flows out the ends of the tubes (which are housed in a shell).
They have filters that filter to 0.1 micron and purifiers that filter to 0.02 micron (yes, that's zero point zero two micron, eliminating even viruses).
http://www.sawyer.com/sawyersaves/products-pointtwo.html
The purifier doesn't flow the fastest, but for bugging in flows well enough, although pre-filtering the water and priming the system help a great deal. The filter flows fast enough to filter 4 gallons in ~3 minutes.
Best thing about this filter? If you backflush it regularly to prevent clogging, it's supposed to last up to 1,000,000 gallons. Avoid letting the filter freeze though, as any water freezing inside the tubes can/will crack/break them destroying the filter's integrity.
For backpacking purposes, I paired the Sawyer filters with the Platypus GravityWorks setup (the GW bags are very convenient). Easily rolled up, filters water quickly and easily and I added 2 Katadyn activated carbon inline filters to the system. The filter takes out parasites and bacteria (the purifier removes parasites, bacteria AND viruses) and the carbon filters take out chemicals, heavy metals and a large amount of tannin (if you filter water from ponds filled with dead leaves).
I used the Katadyn carbon filters because unlike the GravityWorks carbon filter add on, the Katadyn filter is made to be reused. You can open the Katadyn carbon filter and replace the activated carbon with new activated carbon (available much cheaper in bulk online).
Max