For smoothest experience, you should use only single partition (+ probably boot + definitely swap).
You don't have to care about disk sizes. With multiple partitions, if you reserve 10GB for root partition, then it's getting too small if you install few larger programs, and do not clean up log files and so on. That's not really smooth.
Distribution upgrade do not require separate
/home partition. If you want to change distribution (from Ubuntu to Debian or something), then it's useful.
If you fill your home folder with some files, whole system will have problems. My experience says that without software RAID5 or RAID6 you do not have problems with that (you can still boot and then delete files), but it's anyway annoying.
Also, you shouldn't create new folders to root directory. There is Filesystem Hierarchy Standard.
... must never create or require special files or subdirectories in the root directory. Other locations in the FHS hierarchy provide more than enough flexibility for any package.