Font Embedding. The only thing that'll do that off-the-shelf now is Indesign, and well, that's the only thing it does "well".
Macros, search & replace (not just text but styles, fonts, sizes, etc), grammar checker, spell-checker, target viewing application compensation, multiple format input & output, auto paragraph re-justification & hyphenation, multiple language dictionaries, built-in browser, metadata management, multi-style select, discontinuous select, table generation/correction, cross references such as indicies and footnotes, rss feeds....
You said "killer app", didn't you Z?
-bjc