1) Server groups of at least 3 mirrored servers, with a max of 5TB.
This seems like an interesting design choice. What do you mean by "at least"? Does this mean you'll have some data with more replicas? Are these server pools filled up and then powered down until they are needed? How do you choose which server pool to send the data to? And since you have a mirrored set of servers, when do you send a response back to the client?
Is the 5TB number something that is a limit for the storage server (ie 15TB total for a cluster of 3)? That seems rather low. It also doesn't divide evenly into common drive sizes available from HDD vendors today. So what kind of density are you getting in your storage servers? How many drives per CPU, and how many TB per rack? Since you're advertising on low price, I'd think very high density would be pretty important.
2) You say you split data into smaller chunks if it crosses some threshold. Let's suppose you split it into 1MB objects, once the data is bigger than 5MB. And each 1MB chunk is then written to some server pool which has replicated storage (via the mirroring). How do you tie the chunks back to the logical object? Do you have a centralized metadata layer that stores placement information? If so, how do you deal with the scaling issue there? If not, another option would be to store a manifest object that contains the set of chunks. But in either case, you've got a potentially very high number of servers that are required to be available at the time of a read request in order to serve the data.
Just as an example (and using some very conservative examples), suppose I have a 100MB file I want to store and you chunk at 10MB. So that means there are now 10 chunks replicated in your system, for a total of 30 unique drives. Now when I read the data, your system needs to find which 10 servers pools have my chunks and then establish a connection with one of the servers in each server group. This seems like a lot of extra networking overhead for read requests. What benefits does it provide that offset the network connection overhead?
And what happens when one of the chunks is unavailable? Can you rebuild it from the remaining pieces (which would essentially be some sort of erasure encoding)?
Overall, the chunking and mirroring design choices seem to me like they would introduce a lot of extra complexity into the system. I'd love to hear more about how you arrived at these choices, and what you see as their advantages.
In order to not make my long post even longer, I'll not pursue more questions around listings, failures, or hot content.