Well, since there have been 38 views in 13 hours and no responses I'll take a stab at it.
The biggest difference is you don't really have file system level access on an iPad. So you won't be saving stuff 'to the desktop'. You also won't be able to download any file you want (to my knowledge) because of that, but something like a pdf would open fine.
You would most likely be using iCloud for file storage, this makes up for not having a real file system level access. This also means that your files would be accessible on any computer you have set up iCloud with. If you didn't want to use iCloud a service like Dropbox would still maintain commonality between the iPad and your PC.
Flash wouldn't work. But a lot of video sites have video in a format other than Flash. But this does mean a lot of online games won't work. There are tons of free games in the App Store though, so while you won't be able to play a Flash game online, there are still plenty of Flash like games in the App Store.
Office suites will not be as robust as a full suite on a PC, but they would be sufficient for most tasks. You can get a keyboard for an iPad so you could type normally. Apple already has their
iWork suite on the iPad, its very similar in funcationalty to MS Office. MS Office though is
coming to the iPad soon.
You wouldn't need a security suite. Apple has a very closed ecosystem and the apps have to be approved before they are available for download from the App Store. That isn't to say that its bulletproof, but its probably as close to that as you can get.