"High resource usage" refers exclusively to CPU use (the computational expense of your pages and scripts as it relates to time), RAM use (the computational expense of your pages/scripts as it relates to space) and (to a lesser extent) database requests. A poorly-designed script (one that was written with the assumption that it would be the only thing running on a server) can run over the limits with a single user on a "simple" page, or with a single cron run. A well-designed script (one that was optimized for big-O performance in both space and time, that uses denormalized data or stored procedures when appropriate, and that doesn't emit a lot of extraneous markup) can handle a surprisingly large number of users before bumping into the limits.
There are a lot of platforms out there, particularly things like "advanced blogs" (eg., WordPress) and CMS platforms (eg., Drupal, Joomla) that are extremely versatile, but they get their versatility by doing a lot of work at service time. Adding more versatility (using plug-ins, which are not always well-written) can compound the problem. It is unfortunate, but not unusual, to see a simple-looking WordPress site trigger HRU suspensions on every visit -- especially when you consider that the same site, with the same functionality for both the end-user and the administrator, built as a custom development could probably serve a community of a few hundred users quite comfortably.
Storage is limited by other provisions in the agreement (video/image galleries are restricted to 1GB, no file hosting, etc.). Bandwidth is more-or-less unlimited, but keep in mind that the number of users will affect how much computation a dynamic sit has to do to serve pages -- a poorly-designed site will hit the 240 CPU-seconds per hour limit much more quickly than something that was written with efficiency in mind. Emails are covered under a separate restriction (100 per hour max, plain text only, no forging or spoofing of headers) mostly to avoid spamming.