I am biased towards Ingres since I use it at work.
It is cross platfform as it supports many Linux and Unix flavors as well as Windows
SQL92 with propriety extensions but can be tweaked to allow for strict 92 compliance
Many providers (DBD,ODBC,.NET,JDBC) to interact with, as well as embedded SQL with C and a legacy propriety language called 4GL that can create 'green screen' "guis" in Unix which now is revamped into Openroad which can be used to built rich GUIs
ANSI isolation level of course but in version 10 it will incoorporate MVCC (Oracle like) transaction model
Great active support forums and newsgroups. When you post a question you get an immediate reply from their support
It comes in two licenses, open source and commercial. It is being promoted given emphasis to the open source part but frankly I think that it is just a publicily stunt as I don't think that anyone outside Ingres can provide code for the dbms internals/backend
The weak point is that there are no books available;the first book on Ingres (sql) was released last year and it's a good one too. So there is no book like step by step guide or howtos; 5 years back you would excusivelly rely on the manuals but nowdays they release a lot of information online (wiki style,web presentations,guides etc)