If one product defined Microsoft in 2015, it was Windows 10.

After stumbling badly with Windows 8, Microsoft needed to step up with this year's big release.

They did exactly that, with at least three unprecedented moves.

First, they made upgrades from Windows 7 and Windows 8.1 free for the first year. (The only exclusions to the free upgrade policy are corporate customers with Windows Enterprise purchased through Volume License agreements.)

Second, the new OS took shape in the open, with Microsoft releasing a rapid-fire series of preview releases to all comers, through the Windows Insider program. This isn't the first time Microsoft has done public preview releases of a new operating system, of course, but this one was noteworthy for the sheer volume of new builds that reached several million participants.

Third, the Windows 10 development team committed to continuous updates, promising to "keep it current for the supported lifetime of the device - at no additional charge." In July, less than two weeks before the official Windows 10 release, the company updated its support lifecycle page with details. Of course, it wouldn't be a major Microsoft release without an equally oversized serving of controversy, and Windows 10 had more than its share this year. Some of it was self-inflicted, with executives (probably at the behest of lawyers) releasing poorly written announcements that looked like they were hiding something.

But much more of it was just pure sensationalism, from a click-happy press that has perfected a mix of paranoia and technical ignorance so toxic it should have an EPA warning label. With that as background, here's my 2015 report card for Windows 10.

Adoption rate: A

Microsoft set an ambitious goal, publicly declaring its intention to have Windows 10 running on more than one billion devices within two or three years. They're well on their way. Within 24 hours of the July 29 release date, the new OS was running on 14 million devices. A month later, the total was up to 75 million, reflecting pent-up demand from early adopters. That total had climbed to 110 million devices by October 6, which was the last official update of the year. Judging by third-party analytics services, the upgrade momentum has been solid and steady through the end of 2015. As of December 28, 2015, these were the worldwide usage trends from two sources that collectively measure billions of visits per year.

Upgrades and updates: C-

Microsoft has repeatedly described Windows 10 using the phrase "Windows as a Service." That's meant to convey the notion that new features will arrive as part of regular updates, two or three times a year, rather than being saved up for an every-three-years upgrade. That system had its first test with the November update that took Windows 10 to version 1511. The upgrade wasn't trouble-free, and a particularly embarrassing bug forced the company to pull the update from its servers for a few days while it prepared a fix. The monthly cumulative updates are a welcome change, and the November launch of Windows Update for Business eliminated the concerns of many enterprise customers about updates disrupting business processes. Still, there's plenty of room for improvement in the update process and a desperate need to trim the size of major upgrade packages like the one released in November. That should be high on the company's list of priorities for 2016.

Privacy: B

The amount of pure, unadulterated FUD over Windows 10 and privacy was breathtaking this year, and Microsoft >inadvertently enabled its most paranoid critics with dry and legalistic communications. Privacy fears stoke wild (and wrong) accusations this year, like Windows 10 containing a keylogger (it doesn't), stealing files off your local hard drive (nope), and eavesdropping on your personal conversations (seriously, it doesn't). Here's the reality: Microsoft is in the software business, not in the personal data collection business. Windows 10 does collect more telemetry data than any previous version, and the default collection policies are more aggressive than in previous versions. That data is anonymized and used for ensuring that updates work properly and that feature development is prioritized. . As for the rest, yes, your files are indexed if you choose to upload them to OneDrive, so that you can search them. And you can give the Cortana service permission to access your email and contacts list to find appointments and identify the right contact when you enter a proper name. That's not "spying," that's how online services work.

Security: A-

There's a tremendous amount of innovation under the hood in Windows 10, most of it absolutely invisible to mere mortals. Credential Guard, for example, adds virtualization-based security to Windows 10 Enterprise, making it nearly impossible for attackers to steal credentials using attacks such as Pass-the-Hash or Pass-The-Ticket. There are big improvements in multi-factor authentication as well, along with a new feature called Device Guard that makes it possible to lock down a Windows 10 PC so tightly that malware simply can't run. For consumers, the biggest security news is the release of Microsoft Edge, which ditches the legacy baggage (add-ins and ActiveX) that has plagued its predecessor through the years. And full-disk encryption is enabled by default on almost every new Windows 10 device as soon as you sign into a Microsoft account.

Apps: C

Despite some signs of growing momentum at the end of 2015, the Windows 10 Store is still weak compared to its rivals. The biggest reason is the continued huge installed base of Windows 7 PCs, which run only desktop apps. If you're a developer, that's the platform you're going to target. It doesn't help that Microsoft early in 2015 announced an ambitious project to help Android developers port their apps to Windows and then, toward the end of the year, appears to have given up on that initiative. (Developers who already went through the great Silverlight fiasco are experiencing déjà vu.) The availability of mobile Office apps (Word, Excel, PowerPoint, and OneNote) is a positive development, but there's much more work to be done in 2016.

Enterprise support: B

Windows 10 has some superb deployment and management tools for enterprises, and all of its signature software for IT pros (System Center Configuration Manager, for example) has been updated to be compatible with Windows 10. The most eagerly awaited feature, though, is Enterprise Data Protection, which didn't make the cut for the November Update and is now slated for release in 2016. Given the languid pace with which the enterprise approaches major OS upgrades, that's hardly a deal-breaker, and enterprises have everything they need today to do pilot projects in preparation for wide-scale upgrades in a year or two.

Tablets and phones: Incomplete

The selection of Windows 10 tablets is pretty much all labeled Surface, and the dramatic worldwide decline in tablet sales doesn't bode well for the future. At the tail end of 2015, Microsoft released two new flagship phones running Windows 10, but there's still nothing to suggest that the platform has any surprises coming next year that will help it escape from its cellar-dwelling status.