One very important aspect to pay attention to is the advertised performance service you will get. CPU cycles, size of memory, volume of storage, amount of networking bandwidth are all sure to be price points and advertising points. I would encourage everyone to pay attention to any fine print about:
*dedicated vs shared CPU. The biggest problem with CPU sharing is that CPU cycles are scheduled to be shared on over subscribed "cloud" providers, which helps lower cost. Oversubscribed CPU cycles causes CPU wait time, which means that your "cloud" CPU may need to wait X amount of time to be scheduled for your N CPU cores that you are paying for. Let's say that you have 8 CPU's, you may need to wait for 8 CPU's to be unused on the physical host your are on before you get to do any work at all. If you have 1 or 2 CPU's than this is far less of an issue. The greater the core count the bigger the issue.
*Memory ballooning. Memory is one of the most easily over subscribed resources in "clouds". To cut costs Memory is allocated to you at, let's say 12GB. But you only use 6GB. On the back end you are really only given 6GB. Going further let's say that you have 12GB, use only 6GB, but only have 4GB actively in use by your application. There are memory scheme's out there that will write the 2GB that you do not use very often to disk(think swapping intelligently).
*Disk IO speeds. Storage can be really cheap or really expensive depending on how it is architected. Pay attention to any fine print talking about what the storage consists of and if you have any kind of dedicated Disk IO. The cheapest "cloud storage" provider may be offering a product that works great for highly cached low transaction websites. But that same provider may give poor performance for a high rate of disk transaction logging server, or high transactional application.
*bandwidth limitations. Pay attention to quality of service limits. Pay attention to bandwidth sharing, do you get full advertised bandwidth to the internet or do you get "up to bandwidth" limits. Network connections to other servers that are co-hosted could be as fast as 40+GB/s. If it matters to your application ask if there are higher bandwidth connections between co-hosted servers.
*backups, service uptimes, service failure compensation, riders on the contract that talk about lower temporary performance in the event of a hardware failure. Options for expansion of resources(hot or cold).