For me, it was a quilt I fought with. It was lap size size, not really that large, but a million pieces (Bonnie Hunter's Orca Bay, to be exact). And HEAVY. I fought that thing through my machine - figured I'd take the "easy" route and stitch diagonal lines on it. Hah! Half-way through I said to myself "never again", and went out and bought a long arm (Handi Quilter Avante) the next week. Had it in my hands within a month and haven't used my domestic for quilting since. Yes, I did finish Orca Bay on the domestic machine before the longarm arrived.
Of course, I have since realized that diagonal cross hatch on the long arm isn't that much easier
So I've adjusted my quilting patterns to fit the machine.