I understand you still need to drive a lot of server-client communications, saving data in central repositories etc etc.
But what is the primal impact of HTML5 on these platforms when building web applications? Major changes that developers should be vary about?
I believe that the impact on these Programming languages will be minimal to nothing. Because in every language the developers are more and more using frameworks which use the MVC model to develop Web Applications.
The transition to HTML5 only affects the V in MVC. The models and controllers won't change much or nothing. So the only thing that will change a little is the view part of MVC, so the usage of HTML and a lot more of Javascript will change.
IMHO: It will have an impact on HTML and a huge impact on JavaScript (a lot of new APIs), but it won't change a lot in the programming languages used for the backends.
If you're referring to the HTML5 technology umbrella that includes HTML5, CSS3 JavaScript, SVG, Canvas, Web Workers, Transforms etc etc. then potentially a hell of a lot of processing logic will move onto the client, with server-side tech processing a lot of Ajax requests serving JSON/XML.
The server-side will still be very important, you'd still need to validate everything that comes from the client for security reasons, also you'll still be storing stuff on the server-end despite advances in HTML5 offline storage (one of the main benefits of web apps is storing your data in the cloud and accessing from anywhere).
Job wise, you're quite safe if you're more into server-end web development, but if I were you and you wanted to keep your skills relevant I'd seriously look at all the HTML5 technologies coming out, it's all very exciting stuff!
I believe it will have a major impact on your application if you choose to take advantage of the new HTML5 features. The application can always make minimal changes to the UI as in incorporating the new tags, but there is much more that you can do with HTML5 that will affect the ways server-side code is structured.
Here are just a few examples:
Your site can continue functioning even when offline using the new caching features. That means when the client is back online, some sort of synchronization will need to happen.
Web SQL databases can allow storing the data client-side in a structured manner. This can impact the server load significantly. Imagine if StackOverflow.com only pushes down new questions, or twitter.com only sends new messages instead of a document with all messages with each request. That effectively means that the server will play an increasingly important role as a web service which serves JSON/XML and worries less about HTML, Javascript and CSS.
Comlicated calculations which previously froze the browser and were done on the server-side as a reason can be offloaded to the client taking advantage of web-workers.
Web Sockets can be used to replace Comet and other techniques increasing the connectivity between the client and server. However, if your application needed real-time updates, you would already have been using Comet so this change would be minimal.
Image manipulation and processing can be offloaded to the client using canvas.
Besides these, most other changes are view-related and shouldn't affect the other parts of your application as @Jens suggested.
Take a look at these live demos to see what is already possible in modern browsers - http://apirocks.com/html5/html5.html#slide1
The applications will change, as lot of code will run on client instead of servers. That means better and faster application with fast responses and speed that will almost match the speed of an regular desktop apps.