Here's a rundown of noteworthy issues that persist in Windows 10 third-party apps. Many of these will likely be fixed in the coming weeks, either by the app creators themselves or by Microsoft finding and fixing the root causes in Windows 10. But they're worth being aware of as Windows 10 adoption continues to grow.
Applications that aren't high DPI aware
High DPI awareness is likely to be one of the most persistent issues going forward, since it affects many legacy applications and the OS-level fix that exists is only a partial one.
Apps not designed to take advantage of Windows' new DPI-scaling APIs will look blurry when run on high-resolution displays. Many apps that are updated regularly, like Firefox or Chrome, are already compatible. But apps written in the Windows 7 era are likely to have this issue.
The only real solution is to replace those apps with newer builds that honor the scaling APIs -- assuming the apps are still being developed. Alternatively, Microsoft could come up with a better way to scale legacy apps on high-DPI displays, but so far there's no sign of that happening.
Microsoft Office 2003 and earlier
Don't laugh -- for many people using an earlier version of Office, there often isn't a compelling reason to upgrade. Microsoft's official word is that Office 2007 and above are confirmed to be compatible, while older versions "are not certified compatible with Windows 10 but might work using compatibility mode."
Note: That word "might" doesn't apply to Office only, but to any third-party add-ons in an Office installation, some of which may well be even older than Office 2003.
Adobe Creative Cloud and Creative Suite products
If you're using a more recent version of Adobe's Creative Cloud or Creative Suite product, it'll likely be fully compatible with Windows 10. But as with Microsoft Office, not everyone has -- or wants -- the latest version. Consequently, many Adobe Creative Suite products in the CS3 generation have compatibility issues.
A likely source of many problems with Adobe applications is the video card driver. Many Creative Suite apps use GPU acceleration, and some apps that depend heavily on that feature (such as Adobe Lightroom) will experience problems after upgrading to Windows 10. Updating the video drivers after a Windows 10 upgrade seems to fix the problem.
Another thing to keep in mind: Many Adobe product plug-ins may also use GPU acceleration and may not be fully Windows 10 compatible either.
Installers that throw 'Error 1935'
Multiple reports have surfaced about people experiencing an Error 1935 when installing applications -- some of them Microsoft, some third party -- that apparently rely on redistributable libraries of one kind or another. MATLAB, a programming environment for math and statistics, is one program that seems to have this problem.
Some of these issues can be solved by launching the installer for the offending application in compatibility mode, but that doesn't guarantee anything. It looks like a more permanent solution will have to be supplied by Microsoft.
Antivirus and endpoint protection
Most current antivirus products have an upgrade available for Windows 10. But not all of the current generation of free-to-use solutions will work with the new OS. The current version of Bitdefender Antivirus Free Edition, for instance, is explicitly not compatible with Windows 10. (It will install, but the system becomes nearly unusable afterwards.)
In general, don't assume a Windows 7 or Windows 8 antivirus product will be automatically compatible with Windows 10. The best approach is to uninstall antivirus products before performing an in-place OS upgrade, then reinstall only versions confirmed to be compatible with Windows 10.
Any 16-bit apps running on 64-bit Windows 10
This issue's a holdover from Windows 7 and 8, and no fix is likely to come in the form of an update. The 16-bit application compatibility subsystem has been removed in all 64-bit editions of Windows. Anyone running legacy 16-bit Windows apps -- such as custom applications created for in-house use -- either need to upgrade those apps or use the 32-bit version of Windows 10 to run them.
Another possible solution: Install 64-bit Windows but run a 32-bit instance in a VM to run 16-bit apps and nothing else. For those who only need to do this provisionally, consider using the VMs created by Microsoft to run older versions of IE and their attendant OS revisions. They are time-limited to 90 days, but can be reinstalled and reused if needed.