All the charts show graphs with various colours but the legend dialog that shows what they represent should be more easily available, not parked under the Help menu. Add a button in each view to show the legend. The current labels are also needlessly repetitive, just title it "CPU Usage" then "User space", "Kernel", "IO Waiting", 'IRQ Routines"
Also, the numbers shown in the Physical Memory (k) section should be right-aligned and use locale-specific 1,000 separators. Or, would suggest using larger units (MB) like the Windows version does to avoid the unnecessary precision.
Nested processes that show non-zero CPU loads should bubble up their values to the parents so they are still noticable when the child nodes are hidden. The each parent node would then show the sum of the CPU loads of their children.
All numeric values in the main table should be right-aligned as well: PID, CPU, and #thread (should be "Threads")
Inconsistent label in the System Averages section: "last 1 minute..." should be capitalized like the ones below it.