Many old p standard packages have been removed, when their functionality was superseded by new built-in functions. As far as I know, all of these packages are still available for download. You can find the download location through the Standard Package Compatibility Guide. For example, the recently discussed
Graphics`Colors` package is available here. The link is mention on this documentation page.
I remember when the new Documentation Center was introduced, many people (including myself) really missed the old, well-structured Help Browser. It was easier to navigate, and every page had a dependable, fixed location. That sort of clear-cut hierarchy doesn't seem exist anymore. The Documentation Center follows the new (well, in 2015 maybe not so new) trend of using search instead of table of contents. Each page is accessible through multiple (an in my probably wrong impression, sometimes zero) routes ... I'm no longer confident that there is a dependable fixed location for every page ...
You can see all the different ways to get to a documentation page at the top (sometimes this seems to be missing ...)
In response to all the complaints, WRI included the Virtual Book in one of the 6.0.x point releases, if I remember well. It was simple a table of contents. There was also the Function Navigator. Even though I missed the old v5 Help Browser, I only used he Virtual Book rarely. I did find it useful for finding certain advanced tutorials though, or at least remembering their precise location without having to remember fickle search terms to get to them:
In version 10, the button for the Virtual Book and Function Navigator were removed. The Virtual Book is now converted into a standard page in the Documentation Center, but the full hierarchical table of context is no longer visible on a single page, which is slightly frustrating. See this post for details:
The first location displayed on top of most function pages refers to the location within this table of contents.
The old Help Browser is still included as a function component to support old packages that use the old help system. It does not include the main documentation. You can bring it up using e.g.
HelpBrowserLookup["AddOns", "foo"] or by opening the documentation old packages: go to Documentation Center, at the bottom click Add-Ons to bring up the list of installed add-ons, and click an add-on to get to its documentation.