The best thing about open nebula is its ease of use which makes it accessible to all users irrespective their level experience in cloud computing or networking infrastructure domain as well! It also has very good support from community at large who are willing enough help you out whenever required without any hesitation whatsoever!! Not much but if there was one I would like more integration with other tools such us VMWare vCenter etc., so that i could have better control over my virtualization environment using OCS instead just managing them separately via different interfaces/tools available through VMware console itself (which can be cumbersome sometimes).
If anyone wants an enterprise ready tool then this should definitely go into consideration because even though they may not look too fancy initially when compared against some others products currently existing today within market place; however once used will easily understand how easy-to-use.