Hopefully r0ach and others can offer their laymen's summaries.

What you need to know is that Ethereum as it is currently designed can't scale just as Bitcoin can't scale, but the level of scaling which the current Ethereum can do is much less than even Bitcoin's current limitation because verification/validation of Serpent scripts takes more resources than verification/validation of ECDSA signatures.

For both Bitcoin and Ethereum, this is not just an issue of block size limitation. The issue is that in order to scale, the mining becomes more centralized. I think you will should note that Bitcoin and all other major coins are entirely centralized already and on the precipice of failure (all of them! study my links!).

Thus Ethereum proposed Casper which is a design that attempts to use sharding (a.k.a. partitions) to improve scaling decentralized. But I explained in this thread, that can't work. To reduce electricity consumption, Ethereum also proposed PoS-like consensus-by-betting with forfeitable deposits. PoS has known failure modes that violate Nash equilibrium.

So the point of all this is that Ethereum and all the rest of the crypto coins have not yet solved the fundamental issue of decentralized consensus.

If you want to read a theoretical discussion of why, I did that too.

Okay that is enough from me. Adios.