iCloud is a very scary concept since it gives Apple all of your data essentially. Sure, I already do part of it with Exchange server but iCloud security is going to be one of those hot topics this fall when the servers are unleashed to the world and all of those hackers/crackers out there. It may be one of the world's largest data centers with very valuable data. As a "free" service, it reeks of Gmail and Yahoo Mail security. Such low standards compared to an actively managed anti-spam and virus filter on the server end is like comparing a iPod 1st gen to an iPhone 4. They are really just in a completely different class.
So to try and answer the questions, I'll give a few insights.
1. iCloud.com is already reserved by apple so to read e-mails, you'll probably have an SSL encrypted login at
www.icloud.com in order to read your e-mails and login to the rest of your iCloud content. This is the same as MobileMe and Exchange since Exchange had OWA. It's meant as a quickie secure method to retrieve your data without being at a computer or an iDevice. MobileMe's downfall was that it had no offline version that synchronized properly with Outlook or Mail. It was a kind of stupid in-between tech that resembled Intellisync of years past. This weakness in data synchronization is just a poor implementation of their servers.
2. Total data on iCloud is not actually calculated. The 5GB of data is for Contacts, Calendar appointments, e-mails, iWork documents, App data, settings, and Camera Roll. 5GB is actually the standard amount of space when you take a look at modern exchange accounts. It normally stores around 2 years worth of e-mail, all of your contacts, all of your calendar, and a good amount of documents to give you an idea of how much space 5GB really is. The reason why it isn't unlimited space is due to server design and what I think is possibly using an Exchange server type architecture.
Items such as music, apps, books, and photo stream have unlimited amounts of space or have preset limits such as $25 per year for up to 20,000 songs. The other thing I find strange is that it doesn't say video anywhere which means that video storage will still have to be on your Mac or Windows PC.
3. 5GB does not count as centralized storage since I believe that iCloud will split everything. 5GB is the total amount of storage space for items I listed above. The way that iWork applications work would mean that Pages, Keynotes, and Numbers would get stored in their respective places. Any other types of data may not be synchronized but it's too early to tell.
4. Safari can download files just like any other browser or mobile browser out there. Just tap and hold to save your content. If you're saving pictures, it gets deposited in the camera roll and that gets synced with iCloud. Other files may require other applications to open which the "save" process would be delegated to so you can "save" files in an application.
5. I'm pretty sure you can turn off certain parts of iCloud when it gets released. Exchange server works the same way to where I can turn Contacts, Calendar, and E-mail on and off depending on what I want synced. It would only sync with incremental updates so if Apple is smart, they'll use a similar structure since it looks like they are starting to use Delta updates. (Delta = "changes" in mathematic lingo)
