For a long (25mile each way) commute, what kind of bike should I look at?
I would defiantly not rush out and buy a new expensive bike.  Here are the steps you should follow: 

1) Try to make your bike faster.  There are  a few ways that you can do this but I am going to make two suggestions that will be cheap and easy.  


Get slick (smooth) tires.  The should put you back around $60 but should give you a good idea of what a road bike might feel like.  The difference in speed will be shocking to you.  
Put some bar ends on your bike these will help you to change your body position on your ride.  (More positions = greater happiness)




2) Before you buy a "new kind of bike" (ie: road bike, hybrid, etc) make sure that you try it.  I just bought an older road bike on craig's list for $20.  The bike was in very poor condition but it allowed me to figure out what riding a road bike is like.  I upgraded a few weeks later knowing what I really wanted.  

If you don't know what you want I would mess around with your bike a bit and then buy a couple throw-aways.  I bet that you could do that for $50 - $200 (not including resale revenue) but you will know what you want before you drop the big dollars.