"powerful string manipulation functions"
boost::algorithm::split (and the rest of algorithm, frankly) is unintuitive to use. Regex requires looking up the syntax every time. No encoding/unicode support. Literally 100's of popular string libraries, all incompatible, and that's not even counting all the homebrew. char* everywhere so lots of copying to work with string objects (which is what all of the algorithms work on). Dozens of different and slightly different ways of converting other types and object to/from strings. Poor string formatting functionality, and the solutions that exist are verbose and cumbersome (strstream, boost::format, ...) .
I still love the combination of low-level power and high-level abstraction that C++ provides, but string handling is one of the most problematic areas, in my experience (which is of course colored by the type of work I do, but still).