One big problem is that version 6.0 was basically written by someone that didn't understand PSR coding standard properly. With PSR4 in the mind, rewriting it to a compliant module has been one of the main goals.
netcurl is no longer dependent on present curl libraries. If curl is not there, the library will fall back to its simplest mode with internal PHP-stream (binary safe with file_get_contents).
Packages: The library has been split up in smaller parts even if version 6.0 had intentions to "just be one big importable file". Well, that didn't work out well, did it?
In netcurl 6.0, most of the data transmitted in the interfaces was running guessing games which may have caused problems in the way that the IO-parser genereated its own error handler to catch XML errors. This is completely removed.
Furthermore: Packages. The new error/exception handler is shared by multiple packages, and it's in short called ExceptionHandler. This makes it possible to handle detailed errors on a "necessary level".
Errorhandling with SoapClient: There's known problems with SoapClient since 2006, that makes SoapClient unable to spit out errors when credential errors occurs. This happens both in cached wsdl-requests and cached. Version 6.1 handles them both.
Configurables. WrapperConfig is a standalone, where you push in your own requirements. All single wrappers can then simply read from it, by using setConfig().
NetWrapper, the basic old MODULE_CURL implementation supports registration of own drivers in another way than before.
The curlwrapper takes advantage of both "curl_exec" and "multi_curl_exec". By means, if you work with services that only requires curl, you can make several calls within the same session.
A new way to find out which driver/class/method that is missing OR disabled is reached within Utils. The security/utils has separate methods to look for disabled and uninstalled variants.
Version info is not collected from predefined constants, they are mostly extracted from docblocks.
All wrappers can be used directly and separately, while NetWrapper (which is the new primary "MODULE_CURL" class) selects the "best available" driver.
All wrappers now requires WrapperInterface to function properly. With this, noone should be able to miss any important functionality (like authentication drivers).
Response headers are, at least for curl, extracted without the requirement to let curl expose headers in the response.
SoapClientWrapper is using other ways to extract errors, and pushes out more details about this via ExceptionHandler.
Some critical self configured errorhandlers has been removed as they may break other implementations (where exceptions are catched in the wrong place).
SSL is, in curl, using auto detection built in at curl level instead of letting netcurl have a static default value.