1) Replication. Very very few large production Web sites run with one database server. The need to dynamically scale replicas and slave servers is critical for many Web applications.
2) Lock in to InnoDB tables. From what I've seen you can only run InnoDB tables. For some, MyISAM is a requirement, and in some cases provides better query performance.
These two issues right here are why I continue to run my MySQL server infrastructure myself in EC2. If and when they address those two critical components I'll most certainly give the new offering a hard look.
"High Availability Offering — For developers and business who want additional resilience beyond the automated backups provided by Amazon RDS at no additional charge. With the high availability offer, developers and business can easily and cost-effectively provision synchronously replicated DB Instances in multiple availability zones (AZ’s), to protect against failure within a single location."
After having had to contend with replication lag, lots of instances spinning and related headaches for years I'm pretty convinced that scaling reads with binlogs is in my past for good; it's way more expensive than having a good write-through cache.