<p>I've used both and I've found MySql to have several frustrating bugs, limited support for: IDE integration, profiling, integration services, reporting, and even lack of a decent manager. Total cost of ownership of MSSQL Server is touted to be less than MySQL too (.net environment), but maintaining an open mind could someone point out any killer features of MySql?</p>

I've used MySQL in the past and I'm using MSSQL lately but I can't remember anything that MySQL has and MSSQL can't do.
I think the most killer feature of MySQL it's the simplicity. For some projects you just don't need all the power you can have with a huge system like MSSQL. I have an UNIX heritage and find the simple configuration file like my.ini a killer feature of MySQL.
Also the security system of MySQL is much less robust but it makes the job right for most of applications. I believe MySQL it's killer itself from this point of view, and should stay that way, letting young users being introduced to RDBMS with a simple view first. If your project gets big enough that you are considering switch to a more robust system, then MSSQL can pop as a possibility.
That's what happened to me.