From the FAQ:
What is Nokia Account?
Nokia Account uses your email address, phone number or username and password to identify you when you access Nokia applications. You created a Nokia Account when you set up your Nokia mobile phone or when you first used a Nokia application.
What is happening to Nokia Account?
As outlined in our 17 July communications, the Microsoft Devices Group, including the Phones business unit, is aligning its business objectives with the company’s new strategic direction. As we focus on developing hardware that unites and showcases Microsoft services, first-party (Nokia) services and programs for platforms like Series 40, Series 60, others will be transitioned to alternative solutions, where available. The shut down of the Nokia Account is part of this transition.
Why can’t I log into my Nokia Account?
As the former Nokia phones business is integrated into Microsoft, we are consolidating on a common identity management system, namely Microsoft Account. The Nokia Account service will be discontinued on April 24, 2015.
What will happen to the personal data associated with my Nokia Account?
The information you entered to create your Nokia Account (username, password, email addresses, mobile numbers, etc.) will be securely deleted as soon as possible, after the service is discontinued.
Can I continue to use the applications downloaded using Nokia Account on my phone?
It depends on the application. If an application requires a login, you may have to update the existing version of that application or start using an alternative. If some features of the application do not require login, those features will continue to work.
In terms of the impact this will have on existing device users, by platform:
Symbian
There's the Nokia Store, of course - this is keyed to a Nokia account by definition. But then this has been frozen for ages and we knew the Store was going away, to be replaced imminently by an Opera-run alternative. The independent AppList is your friend here, for a route into 2015 and beyond.
Then there's Nokia Maps, for which the favourites are synced through a Nokia account to an online server. The latter will stop working in a few months, but your favourites currently on a Symbian device will still exist, of course - at least until you hard reset or reflash it for some reason. The downloading of offline maps still works and isn't tied to an online account, but there have been no new updates for several years and I wouldn't be surprised if the maps servers were turned off at some point as well. Various third party map archiving projects are underway - watch this space.
Services like Nokia Music will also stop working at some point, though it's perhaps something of a miracle that these still work in 2015 at all, given Microsoft's buyout over a year ago.
Can you think of other services and apps that will be affected come April? Comments welcome.
Windows Phone
Quite a few of the 'Nokia ...' applications on Windows Phone were also tied in with Nokia accounts, though most have already been either discontinued (e.g. Nokia Account, Nokia Xpress) or renamed and transitioned to Microsoft accounts.
HERE Maps is in the latter category, having been switched late last year.
Again, comments welcome on other Windows Phone apps and services that you think might be affected by the Nokia Account shutdown.
Back in 2008, Nokia launched Ovi as its umbrella of Nokia account-keyed services - it was somewhat stillborn, dominated by Nokia Maps, and its various photo sharing, calendar and email services never achieved critical mass. Then came the Ovi Store, later renamed Nokia Store, on Symbian, but this was largely implemented in HTML and not as a native application (as on iOS and Android - and now Windows Phone) and as a result was slow and clunky.
It's tempting, looking back, to say that Nokia never really mastered online services and accounts - there was always a spanner in the works. Microsoft, current transitions aside, does seem to be getting a handle on these, with OneDrive, Office and Microsoft accounts now transcending platforms successfully, albeit at a financial (loss leading) cost to the company.
Overall, I've been surprised that so much has been left working for so long. Between company redundancies, rebranding issues, service changes for users and market confusion, when company A buys out company B there's always mess of some kind and the Microsoft-Nokia story could have turned out worse. Or, of course, Microsoft could have not bought Nokia Devices in the first place and simply supported it and worked with it - but that's a whole other 'what if' for another time.