Hate it or love it, WOW has been a huge success.
Part;y because of it's high customization (ton of different addons, UI options, all kinds of combat logs).
Its dying out only because of the burnout (being very old + stale/recycled content, etc).
the very basic stuff for starters:
- allow UI customization / mods
- Combat logs
- Threat Meters
- DPS meters
IF BW gets this going, it's 90% of the success right there.