A more detailed breakdown of the differences versus high speed rail in general is in this post.
As for versus maglev: maglev is even more expensive to construct than conventional high speed rail, and suffers from the same design challenges that Hyperloop is designed to eliminate. Beyond that, Hyperloop is entirely self-powering - it uses so little power (coasting the vast majority of the time) that it's easy to have enough solar panels atop the tube to provide for its energy needs. Anything not in a rarified atmosphere moving at those sorts of speeds is plowing against a large amount of air resistance.
The small size of Hyperloop cars is a feature, not a bug; it's not just the cross section that's kept down, but the length as well. By keeping cars small (but frequently launched for equivalent throughput), they minimize peak loadings. Viaduct costs are roughly proportional to peak loadings. Elevation allows them to reduce a huge amount of overhead costs (the majority of the costs of a typical rail project) and eliminates a lot of the technical challenges with HSR involving ground shifting and earthquakes, transferring all of your support to readily adjustable fixed points.
As for passenger comfort, the interior looks more comfortable than any train I've ever been on. Of course, you can't get up and walk around, but then again, trips are so short there's not really any need to. I would say that the excellent leg room would be great for stretching out for napping, but that would be a very short nap ;)
As for loading, multiple capsules are loaded up at once. It's not a one-at-a-time thing.