My themes always:
- Main bar that holds most of the theme elements (theme may have the taskbar seperate from main bar).
- Main popup button (rarely used, but looks more complete).
- Task buttons (max-width of 200px, 16x16 icons with full color).
- Tray (16x16 pixels. Only 5 full-color icons shown, scroll buttons on either side).
- Clock (shows Month, Day & Time by default, shows Day of the Week on hover).
- 1px desktop margin on the right side for easy main popup access.
- Matching Rainlendar skin

My themes do not have:
- Winamp controls (I'm an iTunes user).
- System stats (I'm a Samurize user).
- Extra buttons (not needed...hotkeys, Run Dialog and Main Popup do the job just fine. No need for "Holy Trinity" shortcuts.).
- multiple variations (too much work for me :P).
- default background image (I hate themes that pick a background for me).