In terms of padding itself, I believe it can be boiled down into two types of padding, with one being acceptable and one being irrelevant. When I think of padded novels the first name that springs to mind is Dan Brown. He has a tendency to yap on about really pointless side stories or 'facts' to fill his chapters, like spending three quarters of a page on the specifications of a car. This type of padding is just bad, plain and simple; it supplies the reader with no information associated with the story whatsoever. Then there is what can be best described as 'related padding.' This is when you include something that may not be mandatory to articulate your story but supplies information on characters, the scene, and so forth. This kind of thing is actually gives you the word count of a full length novel in my opinion, so it's kind of necessary to an extent. Like the conversations with the estranged, loony brother that @OurJud mentioned -- it doesn't necessarily progress the story in any way but it supplies information about the characters so the reader can get a better sense of who they are. As for 'getting past' a literary agent and publisher, they are business men above all. If they think the story will make them a good buck then they don't give a rats ass if it contains a bit of padding. When crap like 50 Shades of Grey is getting published in today's world then I don't put much faith in their understand of what good literature is anyway.
I sort of agree with @Steerpike -- I'd say it's hard to tell what's padding and what's not, in a novel (or at least, in a good novel). In fact I'd say that's why people like reading novels--you're given a bunch of information, the author tries to show you how it's all tied together, but in the end the work of deciding which parts are meaningful and which aren't is up to you. So I think that "tangents and asides" don't have to advance the story, or even give background information on setting, characters etc. You just never know what the reader will find meaningful.
I believe that a novel is a journey. You can choose to take journeys in a plane, which means you get from A to B quickly, and cover a great deal of ground with minimal distractions. You can take a bus, which means you meander around a lot and maybe stop places you wouldn't think of stopping if you were on your own. This can prove to be fun, and can throw up insights you didn't expect when you started out. You can drive your own car on a motorway, which offers slightly more scenery and distractions that the plane trip does, but, again, the focus is on 'getting there' as quickly as possible. You can also choose to drive B roads in your car (or bicycle) instead, and go more slowly, and take side trips to places that look interesting. Or ...you can walk. Walking will take you ages compared to the other methods of travel, but will be the most immersive experience. You'll see everything up close, in detail, including every subtle change in the weather, the temperature, etc. It will be the most thoughtful and memorable of all journeying methods, and you will get to know the territory you've been through as intimately as any traveller can. But it's slow. Very slow. Some of these modes of travel will not appeal to everybody. Some will not appeal to a single person every time. But they all have things in their favour as well as inbuilt reasons to choose a different mode of travel, depending on circumstances.
