I have been a 40k player for a good 7-8 years, on and off. I played a fantasy demo once about 5 years ago and made up an excuse for why I didn't like it. Actually the truth of the matter was that I didn't like it because the demo was played on a 2'x2' table and there was no opportunity to do anything but "Charge!"
Several years later, GW released the new Ogre Kingdoms book and the new Vampire Counts book, both of which made me turn my head and go "I should buy some of those!" Then I bought just one mini. I already had so much 40k stuff to paint! I was hard to justify buying more models.
So, what would convince you to jump the gap to another GW game, whether that game would be fantasy or 40k?
If you already play both, what convinced you to?