First thing is to define what you want to achieve. Natural ponds are seldom clear all year, have a relatively low biomass of fish, modest external nutrient flows and are essentially transient features of the landscape - there is a natural succession of plants as they gradually go back to the land. Most pond owners want clear water, lots of well fed fish and not to have their ponds gradually turn into a willow grove! The question is, how natural do you want the pond to be, how interventionist do you want to be?
You don't necessarily need filtration. You will need it if;
* You want to keep above the natural biomass of fish your pond can support.
* You want to (significantly) feed your fish
* You want reliably clear water all year round.
* You want moving water (well, you could just have a pump, but you may as well add a filter).
If you are going to have filtration and you want minimal maintenance, there is an awful lot to be said for creating a gravity fed filter with the pump on the clean side of it.
Wildlife ponds need shallow sloping margins for access - saucer shaped. Fish ponds tend to have steeply shelving margins for maximum volume and hard formal edging. You can of course incorporate both areas, with care. An extensive sloping and well planted bog area gives you a nice natural transition from wet to dry, and also provides habitat where the amphibians and insects can shelter from the fish that the frog botherers will tell you that you must not stock in your wildlife pond (but you will anyway). The animals which will appreciate your shallow sloping edges for access include frogs, newts, hedgehogs, bathing songbirds and herons and next door's cat on their fishing trips. This might be an issue if you have ornamental fish you know by name, perhaps less so if you have lots of rudd and sticklebacks.
I think a heavily planted, unfiltered, unfed pond, about 3 feet deep in the middle with an extensive and heavily planted bog garden area on at least one side, lots of surface cover from water lilies and a modest population of small native species would be fairly low maintenance - though as you know well, with aquatic plants the first task is getting them to take and the second is spending the rest of your life hacking them back!
Edited by Steve Walker, 04 October 2013 - 02:30 PM.