I was a Mobile Me subscriber for 2 years. A very happy subscriber, at times it was a little rough around the edges. Although I had very very few problems with it ever. I know personally that after paying $99/year for two years I was a heck of a lot happier with Mobile Me than I am with iCloud. To be frank I feel downgraded.
My problems with iCloud:
Documents in the cloud: basically iWork.com with an iOS plugin. as of yet there is NO way to interface with Pages on Mac OS X, major oversight Apple if my docs are in the cloud I want access to be everywhere, having to download via web to my Mac and then upload again for changes is not the way to go, especially when you have Photo Stream integrated on every platform including Windows I fail to see why you couldn't integrate with Pages.
Photo Stream: (good but feels beta-ish) really no way to delete one photo without wipping the whole thing? Come on Apple you are better than this, it works photos pop up on the other devices in a blink or two of the eye. That I am happy with but I think for once you made it to simple.
Server Reliability: I will be frank I was ****** when I started using iCloud the server wouldn't respond, the process was disgusting to sign in on the preferences plane and then have to manually do the account anyway in Mail. For the first 3 days I still had to tell it to go online because the server was getting to much traffic. I get that they are serving a lot of people but for once my @me.com account was not primary.
I really miss iDisk.