Goals

Call very few external programs (like df/load/uptime/etc), if any. (parse the file system for info, if possible)
Linux version does not use external programs at all and rely exclusively on /proc and /sys and connecting to locally listening daemons. (except for extensions, listed below)
Don't go nuts with eye candy. Don't use ajax. Make viewing the info on text only browsers possible and easy.
Any javascript ever used must degrade gracefully
Don't give info that can be exploited and turned into a security risk, especially not names and command line arguments given to running programs.
Don't use blatantly slow methods such as preg_split(), especially not in loops

Code style formatting

Use tabs for indention. This is legacy and probably should be 2 or 4 spaces instead
Opening curly braces go on same line. Closing curly braces go on their own line.
Look at existing code for more details