"I have read your letter with great interest" sounds more correct to me than if I were to write, "I have read your letter with a great interest".
However a sentence like, "she expressed a great interest in your letter" sounds more correct than if I wrote it without the "a".
Is this just personal taste (both ways being correct) or is there an actual rule which determines when/where you should use "a great interest" vs without the "a"?