I hated when the original Fallout 3 ended. You couldn't go back and visit all the places you missed without reloading a previous saved game. Fallout New Vegas suffers from that as well. Skyrim did not, and it looks like Fallout 4 will not as well.
Bethesda has tweeted out that you can keep playing Fallout 4 even after you complete the main game and there won't be a level cap. Keep raising if you like. This is the way Skyrim works and this is the way Fallout 4 will work.
Now, we can take our sweet time, build settlements, explore Boston, and have a good ole time while not having to worry if we shouldn't finish the last story mission.
In my recent replay of Fallout New Vegas, I visited many places I missed the first time around. There were so many other great side quests and characters to meet, that I was disappointed Obsidian never let you continue after Hoover Dam. Yes, they warned you that there's no turning back, but it sure would have been nice to keep going.
So, it's great to see that with the next Fallout, we won't have to worry about that. Bethesda seems to have included a great many things to keep you busy in Boston and I'm sure we'll get tons of great mods along the way. And, of course, the DLC content that Bethesda is planning on that should keep us entertained even more.
Fallout 4 isn't coming soon enough, but will arrive on November 10th.