Well, I can point to a few things that make for a very bad one
I disagree with all of them.
Use Ctrl-some letter instead of the navigation keys (arrows, home, end, page down, oh, and Alt-v for page up) and Ctrl-d for delete. Use something closer to your fingers than the F1-F12 keys for invoking your own functions. And if your editor won’t let you, get a real editor, one that’s designed for typists first, programmers second 
(smug factor mostly in jest. Mostly.)
It really boggles my mind how people who design editors don’t see the sense in putting the most frequently used commands together in one big cluster. The most frequent are of course self-insert (key ‘u’ inserts self, that is the character ‘u’), and right after that are the various forms of navigation: back and forward by {character, word, line, paragraph, page}, and then deletion and cut/paste (and copy, but not quite as often). It makes sense that you want to use something close to the letter keys for those common commands. Ctrl-letter is the obvious choice (ctrl is easier to reach than alt).
Speaking of keyboards, I can recommend the Kinesis Ergo Elan out of personal experience. It puts a few more keys under your thumbs, which is a big win. Those are: space, enter, backspace, delete, ctrl, alt, home, end, page up, page down. It also splits the non-thumb key blob in two, one for each hand. The columns are vertically aligned (as opposed to a normal keyboard where e is slightly to the left of d etc.), and the keys are placed on a non-planar surface so as to better match the varying lengths of your fingers.
It also adds a second bottom row (corresponding to where ctrl, alt, space, alt, ctrl are on a normal keyboard), and puts the arrow keys and some punctuation there. An interesting feature is that the arrow keys are split in two groups, up/down for the right hand and left/right for the left hand. In my experience, this works fine.
If you’re a numerical keypad addict, you may want to consider mousing with your left hand, since the keypad is embedded in the right-hand part of the keyboard. You adjust to left-mousing surprisingly fast. It’s also a good idea if you use the Dvorak layout: then you have copy and paste accessible when you have one hand on the mouse.
One weakness I’ve found is that some games don’t let you set up your own key bindings and really benefit from having all the keys accessible with one hand (among those are Starcraft and Warcraft III that I know of). Use another keyboard for those, or suck it up, or hack the game to enable shortcut keys.
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