The problem with 30 minutes is that it's not 30 minutes. It's about 18, since you have to get off of the plane which will cost you at least 2 minutes if you are in coach, and you have to present yourself at the gate 10 minutes before takeoff. If you are at the back of the plane, or you have to quickly use the bathroom, now you're at 15 (or help if you both, 12).

Transiting RDU in 12 minutes is easy. Transiting LGA in 12 minutes (high D to C35) is feasible, I just walked from D TSA to C29 in 8 minutes on Friday because I needed the exercise, and I'm coming off of open heart surgery so I'm not exactly speedy.

That said, I don't think that transiting DTW A1-A71 or A1-C18 is doable in 12 minutes for the median traveler.

For me, the biggest risk of a misconnect is the chance of getting stuck with a middle seat on the next flight, something I haven't had in over 1500 flights in the last 23 years. So I won't book anything less than 45 minutes on DL unless it's a transfer at SEA, RDU, or LGA.