Why Won't Johnny Run Linux? - Problems with Microsoft Integration
Can't run many Windows apps

Wine can run some Windows apps ok:
Microsoft Office 97,2000,2003
OpenOffice 2.0
Firefox 1.5
but it has still trouble with
COM-heavy apps (e.g. many VB apps)
Database apps (e.g. MS Access)

Any new app probably exposes a few Wine bugs
Cost to fix Wine for a new app $0 - $250K