Combining things can result either in a charming, lively, personal and beautiful house; or in a confusing, jumbled, schizoid mess.

The challenge for someone like you, who is delighted by so many things, is either to inherit a hundred million dollars and go buy four or five houses, OR, if that's not likely, to put yourself through an exercise.

Gather a whole bunch of pictures of the disparate things you love...ancient dairy buildings in Provence, hillside villas on Italian lakes, sleek penthouses in Manhattan, adobes in Santa Fe, Nantucket cottages by the sea...whatever attracts you.

Then lay them all out all over your dining room table, pour yourself a stiff drink, and look for all the things those images have in common. After a while, you'll begin to notice how each of them makes you feel.

If you love openess, airiness, and clean lines around you, but can't resist a great shell-encrusted rococo mirror, then you want modern architectural backgrounds with one or two big ornate statements.

If, on the other hand, you love heavy classical or Victorian moldings and trims, and flowered prints or paisleys, and you also love sleek Italian furniture and glass tables, then you want a traditional envelope with cool modern art, wool sisal or graphic patterned rugs, white linen slipcovers on simple comfortable furniture, and very contemporary angular floor lamps.

One of the most beautiful apartments I was ever in was a big, open, modern white box with huge windows and dark, polished floors with tribal rugs flung around everywhere. There was a big, long modern robin's egg blue linen sofa with Fortuny pillows, a pair of Eames rosewood lounge chairs, and a really enormous ornate French gold-leafed mirror on one wall. Across the room was a gallery of Italian landscape and Dutch still life paintings and a large Chippendale slant-top desk.

Way way above my pay grade...but an example of how effective careful mixing can be.

As far as mixing metals, I think you can use two in a kitchen before it starts looking unrelated. Hardware and fixtures should probably have something in common, such as period or general tendency to traditional or modern or retro style, or level of simplicity or ornamentation. Maybe three if you have existing vintage brass doorknobs or something.
As far as bathrooms, I don't think they have to match, but again, I would try to have a similar style. Retro in one bathroom and ornate victorian fittings in another would seem chaotic.
Using an eclectic mix of furnishing such as bronwynsmom describes takes a good eye or lots of practice (or trial and error, LOL) to pull off successfully, IMO. I save pictures of rooms I like from blogs and online mags, and go through them looking for similarities. I come up with simple, somewhat traditional or cottage, with warm mid- or light-toned woods, general color schemes of cream, soft green, gold, and a bit of green-blue, natural fiber fabrics, cream/gold or soft blue or green colored upholstery or slipcovers, white bedding, wood floors, seagrass or sisal or tribal rugs, accessories in wood, pottery, glass or basketry, a somewhat lived-in look, and fabulous light from generous windows. Using those qualities as a guide, I can put together a lot of different furnishings and decorative items that fit the overall scheme and "work" together.

I think mixing metals is completely acceptable. It's easiest when one metal is black or dark oil-rubbed bronze that reads as black because those act more like color than metal. However, I have seen warm golden colored metals mixed with cool silver colored metals successfully if there is a nice balance between the two and there is consistency in the application, meaning that all the knobs and pulls are one metal and all of the faucets are one metal, etc.

More than anything, I think rooms turn out best when you have a plan. It doesn't have to be completely fleshed out, but you need to have an idea of the feeling or style, the colors and your general furniture requirements before you start shopping. I've never subscribed to the belief that you should buy a piece that you like with no idea of how to use it unless you are starting fresh and plan to use that piece as the inspiration for your entire design.

It's really hard to talk about certitude in design as I can almost always find an example of a design that works that is in direct contravention to my personal design guidelines.

Bronswynsmom, I have clipped the description of that apt for when I win the lottery. You make it sound like perfection.

wifisker, I really think it all depends on how you do it. In our kitchen we have SS sink and faucet, black hardware on the lower cabs that are stained black and vintage brass light fixtures and hardware on the cream coloured cabs. Oddly, it all works.

I think it's best to shoot for consistency within a room but not necessarily for all rooms when it comes to fixtures. I think lighting can be different as well even within a room.

Our powder room has antique brass Plumbing, but the kitchen has ORB. The bkfst nook light is a different finish entirely, but has the S swirls that are also in the kitchen lights, so there is something that makes them relate.

I like for things to be generally consistent, but I'm not a slave to it. My decorating style is generally traditional with some eclectic touches thrown in. I really prefer a collected over time/things we've collected on our travels look. In my house, the knobs, pulls, and faucets are a mix of chrome and brushed nickel. The chandeliers and sconces are wrought iron (or at least, they look they are!). I prefer the trim and woodwork to be consistent throughout (Generally, I like painted unless it's really exceptional antique woodwork.) except in the case of a den/library/study that's paneled in something fabulous.

This is very helpful. Thank you! I'm feeling a little less freaked out by pressure to choose between A or B now.

My style has always leaned contemporary. But, I used to live near an antique auction house and had tons of fun buying some beautiful antiques and putting them in the home. A new shipment came in off a barge from Europe every week. That was a blast. The antiques I got all worked but they had some commonalities - all were darker wood (mahogany, walnut, etc.) and they had fairly clean lines consistent with my more contemporary style. And, apparently everything looked great because real estate agents were always eager to sell my home(s) because they were "staged" well. (Not multiple at the same time - because I haven't won that lottery! I've just move a lot.) And my homes have always sold quickly.

Speaking of moving - I'm currently building a home and as for fixtures and finishes - it will have two bathrooms and because one faucet I love comes only in chrome I did get a light fixture in chrome to go with it. The other bath will be BN, my go-to metal.

Kitchen - crystal pendants, stainless faucet, BN hardware, and "hazel bronze" pendant in dining room. The pendant is the odd metal out, but needed to pull in the dining table.

My house isn't started, much less completed, but in my head this is all going to look fab together! Hope it is so in real life! =)

As everyone says now-a-days - matchy-matchy is out. But, that doesn't mean chaotic is in! :) Good luck with your selections!

Good advice from everyone.
At the risk of repeating myself ad nauseum, you must trust that you do have an underlying set of preferences of your own, and then set out to work out what they are. The more you look, the better you see.

Saypoint's description of her own conclusions is a fine example. Somewhere under all those things that attract you is a common thread.

Sometimes when we say "Oooooh! I love that!", the question we forget to ask is, "But do I want to live with it every day?"
Keep in mind that what you put in your house isn't a date - it's a marriage.