As mentioned, every person’s choice for this will be different. If you prefer a bifold with a picture window for ID, card slots and cash slots, I recommend the Big Skinny wallet: http://www.bigskinny.net/ . Reasonably priced, thinner and lighter than traditional leather wallets. We shall see how long it lasts but so far it seems to hold up well.
Years ago my chiropractor convinced me to move my wallet to my front pocket. I did backslide a little and have a comb in one back pocket and my pocket calendar in the other but I try to keep that thin and balanced.
When I finally succumbed to the cell phone craze this year I did have to juggle what goes in which pocket so now the wallet goes in the front pocket with the phone and all the scratchy metal items such as keys and pocket knife go in the other.
How much to keep in the wallet is always a balance between utility and thickness but I tend to prefer to include anything that I might possibly need during the week since driving all the way home to get my medical or library card if I suddenly need it is too much of a hassle. Also, not having to think about whether I have the correct card with me for a particular day is worth it.
I keep a debit card, a few credit cards, medical card, FSA card, library card, AAA card, license, Costco card, cash, receipts and half a dozen cards I need for work (ID card, key card, etc.) so one of these ultra-thin wallets does not work for me.
And yes, there are times that I might go to the library on a whim. It is a very pleasant way to spend an hour if I suddenly find myself with time available.