One major restriction is that its platform specific.
In other words, if you buy a Windows version, you can only install the software on 2 x Windows computers. Same applies to the Mac version: install on Macs only.
You cannot mix operating systems by installing on a Windows desktop and a Mac laptop.
Whilst you can swap licenses (Windows to Mac or Mac to Windows), it's still an all-or-nothing deal.
Cloud software can be installed on Mac/Mac, Win/Win or Mac/Win.
Note: If you're planning an upgrade purchase then you must already own CS5. Otherwise you must buy the full version.
One further restriction (not directly related to your question) is that CS6 will never be upgraded. CS6 is the last version for which Adobe will be offering perpetual licenses (pay once and own it forever). So it's effectively abandonware: you can still buy it but it's outdated. It may receive bugfixes, but that's all. Having said that, if it's all you need, then go for it.