One of the most important features is that the backslash key is in the Right Place. Many modern "Windows" keyboards have a short backspace key with the slish next to it instead of below it (where it naturally belongs). Keyboards with emasculated backspace keys usually have engorged enter or return keys (the drawbacks of which are described here). In even more bizarre layouts I have witnessed the slish to the right of a chopped off right-shift.

Taking care of your Model M:

To protect my keyboard against dust and human debris I use a specific cleaning process. Since this takes a few hours I swap out with a backup keyboard for the interim, a more modern IBM soft-touch keyboard. First, remove all the keys (they pop out easily with the help of a screwdriver). This will leave the frame of the board open with the springs rolling around in their pits. Take the keys and put them in the sink with some hand soap. Swish them around for a minute or so and return to the frame. Use a miniature vacuum to clean out the obvious stuff, and then a Q-Tip (cotton swab) dipped in rubbing alchohol to remove the less obvious human grime. Scrub down the frame with a soapy sponge (due to the superior design, you don't have to worry about the electronics getting wet). Return to the sink and finish scrubbing your keys, then drain and dry. Let them sit for a while to air out. Put them all back in place and it's perfectly shiney and new. Keeping a regimen like this I hope to preserve my keyboard for generations. I have never had a mechanical failure in over ten years, but I keep an identical back up in storage incase I need spare parts. When I die, my IBM clicky keyboard will be willed to one of my children.