Is dropbox considered a good option, or easily hackable? Let's say that the wallet is encrypted (talking about export feature in bitcoin-qt).
If the wallet is encrypted with a *strong* password, it can be stored anywhere. If its got a weak password, it shouldn't even be on your PC.
No. Preventing potential attackers from getting access to the encrypted wallet file is a good thing. It is the main reason desktop wallets are recommended over something like blockchain.info my wallet - the attacker does not have access to the desktop wallet file and therefore cannot attempt to brute force it.
If you want to backup your wallet buy a USB flash drive and use that. Or burn a DVD/CD. Don't put it in the cloud.
You cannot brute force a *strong* password. Your definition of strong may vary from mine.