My views on different fields. Note that I have rather limited experience with many of these, so your milage may very much vary, but it is my personal experience none the less.
My limited experience is that game development (compared to many other branches of development) is typically rather varied and quick paced. You can usually move quickly and noone dies if you accidentally introduce a bug somewhere. To me, it's generally fun and stimulating.
My experience with web development is that it's often lots of code/work for simple problems, and it's way to often slight variations of the same problems over and over. Lots of jobs and money involved, but rarely particularly challenging. It's also extremely quickly moving (new game changing tech every week) which may or may not be a good thing (I loved it when I was younger, now it just annoys me).
A huge portion of the mobile app market is also very simple stuff that exists mostly for marketing purposes. If you manage to find a company that focus on a particular app/service instead of contract work however, you might be in for a really good time.
Application/tools development is highly dependent on the actual product. Some are super fun and awesome, others are horribly over engineered solutions using extremely outdated and aweful tech that's just painful to deal with. If you can live with VB6, COBOL or FORTRAN, there'll likely be a lot of very well paid positions opening up in the not too distant future.
Systems programming (OS, drivers, robots, vehicles etc) can be pretty cool, but you'll probably move painfully slow due to extreme demands for testing and code review.
Small target embedded development can be pretty cool imho, since you work at such a low level. It's like being thrown back in time 20 years. You usually work strictly with C and assembly, and the targets are so small and slow that you'll actually have to think about what you are doing. There's a lot of frustration involved with bad tools and stuff though, but it's getting better.
My current day time job deals with PC tools for embedded development. I got this job largely thanks to my experience and interest in game development, and my initial task involved optimizing an application using technologies and techniques typically found in games (ie using the GPU for heavy work).
