I've been battling this question myself.
The important question to ask is "what do users interact with?" Literally, what do they see on the interface? They interact with "wallets", of various kinds. They interact with "transactions" and "labels", and "confirmations", and "transaction amounts". The names need to start simple, and have a hierarchy that accommodates the various gradations of user education.
Let me compare it to iTunes. What would a user say he interacts with?
Would a user say he interacts with "media" and "metadata" and "album art" and "playlists"?
Or would he say he uses iTunes to listen to his music, and to organize it, and load up his iPod?
The moment we decide that the user even needs to know he is "interacting with a label", the resulting software is no longer speaking his language.
I submit that the user should be able to use the software without needing to name anything.
You can't just call everything a "wallet" with no qualifiers, because that neglects the profound differences between different kinds of wallets.
Let me compare it to vehicles. What if I told you you can't just call everything a "vehicle" with no qualifiers, because that neglects the profound differences between cars, buses, trucks, and trains? Not to mention airplanes?
On the other hand, if I say "vehicle" to you, you probably think of a car. This is probably because cars are the most popular kind of vehicle that you and everybody you know interacts with.
When deterministic wallets are the norm, the word "wallet" may as well mean one.
For users that use nothing more than "online, maybe-encrypted wallets", using "encrypted" or "unencrypted" is all the qualifier they need.
If I sent you a PDF file and it needed a password, and I called it a "password-protected PDF", you would get it. So would the average user. So this is entirely acceptable as a qualifier.
The most important thing here is it is a distinction that is meaningful to the user. 100% of users will care if a password is needed, since it's instrumental to their ability to use the file.
But once you go beyond "standard" usermode, users have options and need to understand what those options are. And that's a million times easier if there's consistent names between the applications giving them these options. Armory uses deterministic wallets, Bitcoin-Qt uses "loose-key" wallets -- the user should care that "loose-key" wallets need to be backed up regularly. In this sense, anything that will show up on the user interface to the 80th-percentile-and-lower user base, should have simple, unique, fewer-syllables-preferred names.
The average user would be satisfied calling it a ".dat wallet", a ".bw3 wallet", or whatever.
If the user has to stop and ask "wtf is a loose key wallet", you've lost his mind share. 99% of people never ask what does "MP3" stand for, this doesn't stop them from using their iPod.
I would strongly suggest that different kinds of wallets be given names that have no words in them, and leave the job of explaining the technical differences to the documentation.
I accept .wallet as a good extension, but this should be reserved for a wallet type that is a) standardized, b) well accepted by the community, c) has all of the important features we have determined that wallets should have. At the very least, a user should be able to expect that wallets saved by any application that creates .wallet files can also be read by any future application that reads .wallet files.