Ok. Here's the truth about mainframes. I have been working with these wonderfully powerful machines for over 25 years. You can have an extremely rewarding career, if you do your homework.
First of all, a mainframe is extremely good at processing incredibly large amounts of data very quickly. While servers have typically grown up from single user systems that had multi-user capabilities slapped on, mainframes have had multi-user functionality at their core since April, 1964.
Second is that mainframes are extremely robust in their ability to provide RAS (reliability, availability, scalability). Reliability means that they are not as susceptible to hardware failures or badly written programs as servers. Hardware is highly redundant and has autonomic healing capabilities that servers just don't have.
Availability means that the system is there when users need it. Some mainframes run for years without needing to be IPLed (rebooted in server parlance). When they are IPLed, it is an extremely controlled event and is usually only needed for the most extreme reason. Individual address spaces or functions like CICS and DB2 need to be recycled from time to time to get new features or fixes in, but the operating system and the hardware are usually not impacted.
Scalability is my personal favorite. An application written to support a single user does not have to be rewritten to support 1,000 users. In fact, the more users an application has, the better the system generally runs.
Mainframes are big, powerful and very sexy machines. 70 percent of the world's data is housed in mainframe processing complexes. You don't have to have much more than a little bit of Windows or UNIX knowledge to work with servers, but you actually have to know what you are doing to work on a mainframe. It separates the adults from the kids and the wannabes real quick.
So go ahead and play with your GUI interfaces and your spinning logos that play cute tunes. I'll be in the data center making sure the mainframe is processing 20 billion financial transactions a day or the entire payroll for a 100,000 person organization or managing refugee care for hundreds of thousands of victims of a natural disaster somewhere.
By the way, the largest Massive Multiplayer Game Environments run on something called a "gameframe". You guessed it: it's a mainframe running Java or C++ on Linux. And it's making sure that Halo 3 and World of Warcraft provide gamers the silky smooth immersion experience they are paying for.
Remember: if you can lift it, it ain't a real computer.