HTML5 will reduce the importance of plug-ins
Once upon a time the Web world liked the idea of a browser plug-in or add-on because it encouraged creativity and experimentation. Sounds, moving pictures, and other neat tricks appeared on the Web first through plug-ins built by Sun, Adobe, RealAudio, Microsoft, and many others. The plug-in interface was open to all, and everyone experimented with adding new features to the old, text-based world.
Will the idea of a plug-in disappear or fall into disfavor? Perhaps, but it depends on what you want to do. If drawing images is your goal, then the Canvas object may be powerful enough. But if you want to build specialized 3-D worlds like the ones found in the more sophisticated Flash and Shockwave games, you may be pining for the old days when a plug-in could get direct access to the video hardware or run a 3-D game world.
HTML5 will enable more interactive graphics
The old Web loaded images by downloading a GIF or a JPG file. The new Web can build an image on the fly in a Canvas object. A number of good graphing libraries have appeared, and all of them make a Website's graphics much more interactive.
There is a legitimate danger that all of this sophistication will overwhelm the poor client-side processors. In the past, some developers deliberately disabled the Flash plug-in to avoid the headaches and overhead of rendering heavy Flash content. That won't be an option in the future. Everyone who's been complaining about Flash may learn that the troubles had little to do with the technology itself -- the problems came from the designers battling for our attention.
HTML5 will allow applications to tap local file storage
Web programmers have always been able to store a surprisingly large amount of information in cookies (300 cookies of up to 4,096 bytes in IE), but to do real work you need more room. The early versions from the Dojo toolkit used the Flash plug-in to commandeer a section of the hard disk, but now the tools can simply use HTML5.
The technique does not need to undermine the hard work of cloud proponents, though, because the local databases can act like smart caches. Game programmers might store descriptions and artwork locally, saving the time of downloading the information again and again.
On the downside, these databases are buried deeply in the system folder, so making backups may not be the simplest step. Users who may want to move their local data from machine to machine will pull out their hair. Or perhaps we'll just see a hybrid cloud/local approach appear where the local machine caches the data but the cloud maintains a definitive version that can be accessed from different machines.