I hope I'm not making a re-post, but I could not find a corresponding topic, anyways let's give out the rules: 
 
This topic is intended for players not having played the game or in the middle of playing it, hoping to learn a good place to stop progressing with the main storyline not to hit the ending before having explored all the cool stuff. I mean, I would love to hear a good stopping stuggestion point to scour and explore the wasteland, before finishing the main story and getting to see the ending. As the game is like Fallout 3 before patching (Fallout 3 vanilla?), the strict end kinda bugs me and I would love to explore the game as much as possible, before letting the ending give the judgement for my character (if there is one).
 
ie. (Fallout 3 spoiler!) 
 
 
  
 
 
 
 
 
 
SPOILER!!!! 
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To stop before leading the assault to Water Purifier with Brotherhood of Steel and Optimus Fallout. 
 
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SPOILER!!!  

 
 Please, anyone having completed the game responding to this, no spoilers and try to keep the answer as a hint. Kinda like ppl do with GB quests. Be creative :) 
 
There isn't really a "right answer" to what I'm asking for, I don't necessarily want to hear the last possible step before the game ends, but rather suitable points in storyline are welcome. 
 
Thanks already in advance and I hope everyone is having as big blast with the game as I am!

You will be given a notification asking you if you want to start "the end" of the game.
 
The ending is final, so no playing after it.

" @JustTheDoctor: Oh, so there is a clear "wanna do it now" moment? Guess they learnt at least something about FO3. Pretty definite answer, feel kinda silly about making this post now. Gracias señor :) "

  Or you could just play through it a bunch of times like I'm doing. "

I stopped following the main story quest line once I arrived on the New Vegas Strip. From there I went off and just explored and side-quested my heart out. Any further down the story quest line and you will start making major decisions about how you want to shape the end of your game. A few are reversible (to a point), but I found it a good place to stop before committing to one particular endgame scenario.

In my game, it was fairly explicit about the point of no return for choosing a faction to support.  I started getting messages saying "If you keep helping X, then X will be angry" as I was going down one faction's main questline.

I've come at a point where if I help a faction further I will fail a shitload of quests for other factions. I did an NCR quest and failed 10 Legion quests and then reloaded a save. If you try to please everybody like I do, you will probably get to this point and that'll be a good stopping point. After I am done with the side stuff (gathering unique weapons and so on), I will make a "master save" and branch out in the difference factions so that I can get the achievements without playing the whole game over again.