There are a few things that some people seem to be missing the point about.
Megaton exists as an homage (like many, many others in game) to the bomb worship in "Beneath the Planet of he Apes" (I think it was Beneath, anyway... been decades since I watched the movies). If we want to question plausibility, we would need to ask why Cromwell is still alive (he'd be dead in minutes, at most, standing in that pool of radioactive water).
We also need to keep in mind that the world of Fallout is a projection of what the general 1950s American populace believed that the future might be like. New information abouut radiation behavior continues to take place today, and major discoveries happened during the 1970s and 1980s (quarks and associated subprotonic particles, nuclear winter, etc).
As far as an "empty" wasteland, I would have to disagree, especially if playing with MMM for FO3. In fact, just the opposite is true. There is no way that tons of wandering NPCs and creatures would be alive in this world because there is no way for the ecosystem to support so much life. The Mad Max movies and similar stories had a more realistic portrayal where you almost never found life in the wasteland, and the remaining humans tended to group together for survival. Another way of looking at it is that solo individuals or small groups would tend to die fairly quickly from one of many reasons, so all that would remain is the larger groups who could survive together (larger being relative, of course).
Overall, I find it ludicrous that it is quite difficult to simply walk around one group of enemies without running into another group in a nearby location. It's very unrealistic and becomes a waste of time.
Oblivion had exactly this same problem, by the way. It was ludicrous in Oblivion to run into bandits and other monsters on well guarded (supposedly) and well travelled (supposedly) roads in the Imperial province. The fact that it would be impossible for the economy to support itself is proven every time a travelling merchant meets one of these encounters and is killed with little trouble. So much for safe trade routes.
Finally, the carry limit is beside the point just as it is for an FPS. FO3 is a hybrid RPG/FPS, so it wouldn't make sense to have a carry limit (aside from the encumbrance which even FPSes do not have). It's interesting that no one complains about the silliness of Gordon Freeman or other FPS protagonists carrying around a couple of armories' worth of weaponry and ammo.
In the end, to each their own.