There is no such thing as a commuter specific bike. The answers to this question alone illustrate that. One person lists the must haves in his view and another blasts him for being presumptuous. The must haves vary with environment and the length of the commute. For example, my must have is that it must fold. If it does not fold it is not a commuter bike for me. That is inviolable, imposed by railway rules. Obviously very few of the others who post in this section feel this way. You have to fit the bike to the commute and with a 30 mile commute I think you have to look at a road bike or a performance hybrid which is very nearly the same thing. If the route you must use does not allow something very much like a road bike then maybe you should look for a different transportation option. Doing 30 miles twice a day over poor roads on a MTB will certainly build you up but it will be very tiresome over the long run, I would think.
As others have said you cannot turn your brain off while bike commuting. I don't think you can turn your brain off while cycling, period. It is sad really but when I cycle I think only of cycling and it has to be that way. It would be nice to forget for a while what you are doing and bask in the scenic wonders you pass through. You are moments from a spill or worse if you do that, you have to pay attention to what you are doing. You can spare brief glances and that is about it. When you are commuting you have to pay attention to the traffic because the traffic pays no attention to you. I'd have been run down twice so far this year by people driving on the wrong side of the road if I had not been paying attention! So your "commuter specific" bike will be one that fits the road surfaces you travel and it will have whatever other features that are needed or helpful for your commute. It may or may not resemble someone else's ideal commuter. There is no need to worry if it does not. The one thing it will not do however is allow you to turn your brain off. Now riding partway on the train does allow you to turn your brain off for a while, just be careful you don't miss your stop.
For the person who asked about group rides I would say if you can keep up on your commuter you can ride your commuter. I don't take group rides but the impression I get is that it is all about keeping up. It is something I might do in the future and I am certainly not going to let the rest of the group's prejudices of what bike to ride dictate my choice. If I can keep up with them on the bike I am riding I might take some ribbing but it won't have much force. If I can't keep up, I'll just look for a slower group because I don't think my bike or its accessories prevent me from keeping up with anyone. It's the engine, it's always the engine....
Ken