In reply to Gary.

For not showing captions, you can simply not add a caption to an image in the gallery. But, I'm guessing you have some situation where you have old captions or something that you want to hide. Using CSS or a custom filter is the best way to do this at the moment. It's probably not a common scenario, so that's likely the reason there's no option.

For aligning a mix of portrait and landscape images, the new image size option can work really well for this. By choosing a specific cropped size, all of the images can align in a grid, which is on par with the original gallery shortcode.

As for the gallery in Jetpack, I believe it uses the Masonry script or something similar. You likely won't see anything like that in core WordPress because of the extra script load required to make that happen. Generally, this is considered plugin territory because this extra script slows down page-load speeds and would be unwelcome by many. For features like this, it is really ideal that plugins or themes handle it. That way, users who wish for that sort of functionality can have their pick from the available plugins. It also keeps WordPress from tying itself to a specific third-party library on the front end, which is generally a good thing to avoid in the long run.