The best way to make sure they (we, since I work at Microsoft) get the feedback is by finding out where the product is listening.
A lot of products have UserVoice set up, a lot of dev product teams (dotnet, asp.net, etc.) are looking for GitHub issues.
For Windows 10 I'd recommend using the Windows 10 Feedback Hub - if you hate something about Windows 10, post feedback in the Feedback Hub before you uninstall it.
Visual Studio has this page that explains where best to submit feedback / bugs / feature requests: https://msdn.microsoft.com/en-us/library/b8akas30.aspx
There are lots of us who see your comments on HN / Reddit / Slashdot / etc. but having been part of the Microsoft ecosystem (both inside and out of Microsoft) for many years, I can tell you that there's a huge difference between "I've seen some unhappy comments about X" and "Our feedback numbers show 68% of users want us to improve feature X". Also, specifics are important - a detailed issue report with repro steps and recommendations for what you'd like to see work a whole lot better than "@drunkvs LOL VS suxxxx ikr!!!"
Also, just because I'm an ocd fact checker, Ballmer technically didn't say "open source is a cancer". He said that Linux is a cancer, referring to the strong copyleft license requirements (https://web.archive.org/web/20011211130654/http://www.suntim...). [yes, I very well know that you can run software on Linux without that software being GPL. I'm writing this from Red Hat DevNation :)]
Regardless, I can tell you that our current CEO and everyone I work with is excited about writing great software that runs everywhere.