And why is there barely a changelog regarding this pretty significant OS version?
Depends on what you mean by "significant". In my experience as a programmer for Apple products, each .1 release is a major bug fix release, and a few APIS come to surface. These bug fixes usually don't make it to public change logs, but they do exist.
Just speculating here, I haven't installed any beta, but I believe that 5.1 will resolve the remaining battery issues, bring many bug fixes to the table that few will notice, and maybe we will see even better performance.
Personally, I am not one of these people that are eager for the next release. The only releases I was eager for was iOS 3.0 for implementing Core Data, iOS 4.1 that had great performance gains over 4.0, iOS 5 for the even greater performance increase and some new APIs, and OS X Lion for countless functional reasons, including performance on my 5 year old.