The C++ standard library is probably the highest quality standard library of any language I've seen.
It is documented down to an very low level. I can't count the number of time I've been using some .NET library only to find out that it has some undocumented requirement, quirk, or wildly unexpected time complexity. You never get things like that in the C++ standard library -- assuming you've read the documentation thoroughly, you should never be surprised.
The standard library takes full advantage of the language, and it's as lean as ever with the "don't pay for what you don't use" mantra generally remaining in full effect.
A downside? I may be able to develop something that uses a tenth the RAM and half the CPU in C++, but despite the strengths mentioned above, it's going to take me at least twice as long and I'm going to need to juggle a number of inconsistent 3rd party libraries -- no doubt some of them being plain C so I'll need to make some RAII wrappers, etc. -- it remains incredibly low-level.
Boost picks up some of the slack, but C++ really needs more of the things commonly used today. Things like HTTP, XML, web services, SQL, configuration, and cryptography should be built in, but they're only just now looking at a simple sockets library. This is a huge weakness. C++ is used in a lot of low-level situations so I don't know if these should be part of the standard library proper, but at the minimum an additional "framework" standard that implements high-level stuff for the more common unconstrained users would be immensely useful.
The language itself is very strong, and C++14 cements even more useful things into it. The only things I wish they'd add is LINQ and async functionality similar to C#.