Well, this is my idea so dont quote me or anything... this might be wrong.
First of, lets leave netburst out of the question because it just sucked to begin with.
When you look at Athlon 64 vs Core 2 it might be a little easier to explain. First off, A64 has a bigger L1 cache (64kb I and 64kb D), so that helps it at that level. Next, the A64 had an integrated memory controller, so it didnt really Need (or benefit from) much cache, because memory access was so quick. The memory controller (and bigger L1) might explain why the A64 DIDNT benefit much from extra L2.
Now, Core 2 has smaller L1's (32kb I and 32kb D), so it needs to make up for that somewhere else, which is does with the L2 being a good bit bigger to compensate. Next, it doesnt have an integrated memory controller, so its memory access is (relatively) slow. So, when main memory access is slow, it helps a lot to make up for it with much faster L2 memory. This MIGHT be why Core 2 benefits so much from extra L2 cache.
Note, that everything I said is simply an idea, I am probably wrong, so dont yell at me if I am.