I've been working with MySQL 5.x for several months now, and I'm confounded by the fact that the datetime data-type does not allow us to include timezone information.
For my current project this has been a big deal as I am in the process of adding in the ability to handle multiple timezones as part of our efforts to take EchoTools global.
For us, our product and servers started out based in US Central Timezone. Had MySQL been capable of storing the UTC offset, it would make a lot of our conversions much simpler. As it is, we must take special care whenever a user enter's a date, say to schedule an email drop, to ensure that the date and time are converted from their timezone to our server's timezone and stored.
It's a bit more work than one has to do with some more mature database engines. Is it horrible, no. Just something to be wary of.