Ditto what Nick said, mainly about our linebacking corps - which just happens to be the most important part of a 3-4 D. To run a successful 3-4 defense, you need big linebackers and nearly every Ram LB outside of Chillar fits the mold of a smaller, WLB-type.

I'd rather stick with what Haslett is running now anyway, as I think being able to alternate occasionally to the 3-4 alignment (if it's put in at the right situation) can cause more confusion and brings more to the table overall.