Here's a look at what you might call three different "cloud personalities" that can help you decide what you want to get out of a service like Google Drive, OneDrive, or Dropbox. I've also included some suggestions about services or strategies that might work best for each type.
Convenience cloud
This type of person who isn't concerned with issues surrounding security or privacy like hackers or governmental snooping. Their primary concern is simply having access to their files across all their devices.
The fact that most cloud storage companies secure downloads and uploads via HTTPS and keep personal files in a secure manner in their data centers is enough reassurance for these people.
If this sounds like you, then you probably shouldn't worry too much about what kinds of files you put online. But even if you're care-free you'd still do well to keep any personal financial information out of the cloud--just in case.
Care-free types would find OneDrive a good choice, since Microsoft offers unlimited cloud storage to Office 365 subscribers paying $70 or $100 per year, or 15GB of free storage (with the option to pay affordable prices for more) to non-O365 subscribers.
Lockdown cloud
This type of person is more wary of the cloud after the Snowden revelations. Constant stories of hacking also remind the secure types how vulnerable online information can be to exposure.
There isn't really a perfect answer for the deeply security conscious--at its core, cloud storage is all about storing your files on somebody else's server--but a good solution might be to encrypt files yourself before they get sent up to the cloud.
The thing about self-encrypting files is that you have to be careful not to have an encrypted file open on two devices at once. That could end up corrupting the file. You also have to figure out how you're going to decrypt files on your various devices. If you use something like miniLock, for example, you'll be restricted to accessing your files on PCs.
Secure types will also want to watch out for features on their smartphone that automatically upload photos and other data such as passwords to the cloud.
Security and convenience
Finally, we come to the last example, which is a mix of both convenience and security that I'll call "only the essentials." This is someone who wants to make the cloud convenient, but also wants to take at least a few precautions against hacker intrusions and other potential online mishaps.
A good starting point for this type of person is to decide what kind of files you absolutely need to access on multiple devices, and only store those crucial files in the cloud.
Next, you'll want a cloud storage provider that uses encryption but leaves you with the only key to unlock those files--typically your password. A good choice is SpiderOak Hive, a Dropbox-like service from SpiderOak that offers 2GB for free of encrypted cloud storage (and more for a price, of course). SpiderOak is a so-called zero-knowledge provider because they will never know how to decrypt your files--only you will. The natural downside of a zero-knowledge service is that if you forget your decryption password, SpiderOak cannot retrieve your files for you.
SpiderOak is available on PCs and mobile devices.
There you have it: three basic cloud types. If you're not sure about what you want to put in the cloud, thinking about which cloud personality you fit into can help.