Ugh...
For a moment I thought that at least this time Bethesda would look to the original Fallout games (or, you know, New Vegas) and decided to not go the Bethesda-route and offer a classic Fallout-ending where you get shown the consequences of your actions and that's it.
But noooo, of course it's going to have an open ending with post-ending gameplay. And no level cap so you can make über-characters with every perk in the game. Fuck this.
You know, before reading the reasoning behind why you couldn't continue playing New Vegas I actually preferred the let you keep playing. Mainly cause I prefer to finish the main game then do DLC rather than have to do the DLC then main game.
It's not a deal killer for me like it seems it is for you, but yes I'd prefer to hear it has a definite ending (Even if they did it Witcher style where you can continue but it's considered playing before the ending). Mainly because that would mean they could have a lot more variance in endings. Otherwise it would be too costly to try to have the game diverge enough to accommodate every ending if they had very divergent endings but let you play afterwards. Having it be playable after you finish the main quest says to me that your actions in the end won't have too drastic affect on the world. Or it will only have one type of drastic affect on the world (so they only have to change the world in one, maybe two ways after the game is done). Or they just ignore the ending entirely (hey, you could win the war for the stormcloaks in Skyrim and it didn't seem to affect too much. YOu still even got soldiers marching stormcloak prisoners around even though supposedly the stormcloaks had kicked the imperialists out and won Skyrim).
I don't mind the endless leveling though, that doesn't really bug me. I always worry I'm going to reach level cap before I'm done anyways. This will remove that worry for me.