I've been through a similar experience that unfortunately ended up quite badly. Here's my account, I hope it helps the discussion.

I was on a 2 hour flight, we had left maybe 15 minutes before when the passenger sitting right in front of me started feeling unwell. He was travelling alone, so the passenger sitting next to him notified the flight attendant. The flight attendant asked whether he had a history of diabetes, but he was in such pain that he could barely articulate a word. He would only say "it hurts". Believing it was an episode of hypoglycemic shock (not sure based on what, but well, IANAD), the flight attendant brought him a glass of soda and, maybe five minutes later, seeing no improvement, shouted the usual "is there a doctor on board?" question. There was indeed a doctor on board (two, actually), who immediately proceeded to examine the person (he even had a stethoscope). He asked for the emergency kit as well, which proved to be quite minimal.
As the flight attendant debriefed the doctor (who at this point, still believing the thesis of hypoglycemic shock, tried to measure the heart rate in parallel), the patient collapsed. It was then clear to everyone that he was experiencing cardiac arrest. The doctor quickly put together, with his other colleague, a small group of people that manage to lay the person down on the aisle floor. In the meantime, the pilot was informed of the situation and diverted the plane to the closest airport en route. By then, at least 20 minutes had passed since the start of episode. CPR was at first successful, but the patient lost his senses again after a few minutes. A second attempt at CPR proved unsuccessful. We landed maybe 30 minutes after the pilot got to know about it. The ground medical team hopelessly tried reanimation with a defibrillator (there was none on board, BTW), nothing.
It was pretty sad, especially because I have the feeling that if that person had been on the ground he would have almost for sure survived (IANAD, once again, so, it's just a feeling).

Anyway, things that went awfully wrong and are a danger to airline passengers:

* As I've said before the first aid kit was pretty basic and, according to the doctors, lacked some essential material;
* Most commercial planes aren't equipped with a defibrillator. They're not mandatory, at least not in Europe.
* Airline crews know nothing about first-aid. They're just not prepared. They cannot recognize the simplest symptoms of a heart-attack.
* They clearly didn't know what the first aid kit had or hadn't. They couldn't name the contents. To be fair, the crew was french-speaking while the doctor was not, so maybe they just didn't know the names of things in English. Still, crews on international flights are supposed to have a good level of English.
* Most flight attendants behave like robots in stressful situations. They have such a respect for protocol and rules that they will be reluctant to break them even if that means saving a life. For instance, as the doctors were applying the last round of CPR, they considered improvising a tracheotomy using a pen (desperate measure, but who knows whether it would have worked?) By then the plane had started descending and was maybe 10 minutes away from landing. As the doctors asked for a pen, the crew remained still in their seats.

Bottom line: if I ever have a heart attack (which I hope I won't), I'd better not be on a plane.