So why is this free?
What am I going to find out later that makes me mad I can't revert back to 7 after a month?
John Jones explanation above is only part of the reason, I think. There are lots of theories and I don't think Microsoft has confirmed any of them. But here are some ideas:
1. Widespread adoption of Windows 10 provides a platform for them to sell aps to. They have already made lame attempts at this with their attempts to sell solitaire. I think they'll do better at is as time goes on.
2. The widespread adoption of a single platform will reduce their support costs.
3. It leaves open the possibility over the long term to charge people for regular updates.
4. As John Jones explained above the failure of the world to adopt Windows 8 was a festering problem for Microsoft. The Windows 8 debacle lost them market share and it must have really pissed off their partners who probably sped up efforts to get non-Microsoft platforms out the door.
5. Google already gives their OS away for mobile aps. It would be pretty hard for MS to compete with that if they continued to charge for theirs.
6. MS can make a few bucks by controlling the default applications for some things. More installed OS's the more MS can make from this kind of thing.
7. One of the grand strategies behind the Windows 8 debacle was the Microsoft dream that people would think it nice to have the same OS on their mobile devices as their desktops. MS has not abandoned that idea and their theory is probably more Windows 10 installs equals more people buying MS mobile platforms.
ETA: This is related to item 4, but widespread adoption of Windows 10 on people's old machine will make it more likely that when people buy new machines they will by one with an MS OS and MS is still charging for OEM licensing.
As to what is going to make you mad in the future about MS and Windows 10:
That's an interesting issue. I'm already disappointed that they chose not to support Windows Media Center since that is the heart of our home entertainment system. I have Vista on my desktop and MS won't update Windows Live photo gallery and they blocked my version of Vista from being used as a Windows Media Center extender.
So overall MS is often not very nice. They failed to own up to bugs that make Vista update a horrendous experience. They take stuff away when you update the OS and some of that might piss you off.
So what do people think MS is going to do to piss off Windows 10 users as time goes on? The automatic update issues are already pissing off quite a few people right now.