In addition to pivot tables do not forget trying to do anything with MS Access. I have also noticed that opening powerpoint presentations can grind LO to a halt. Most recently the Stream Ciphers lecture from Boneh's class took forever to open in LO on a 6 core AMD with 8 gigs of ram...

I agree that most of the time opening .doc/.docx with OO/LO is problem free. But you never used StarOffice, back in the day things were not always so easy:) Plus StarOffice was not guaranteed to stay open;)