Can you be more specific?...I'm still early in the game but think it's pretty good so far, I've played all the previous Fallout games so know more or less what to expect.
I think learning curve is a bit higher in this game especially the crafting aspect of F4.
I think the real disappointment is that it isn't improved/novel enough compared to FO3 and NV. The crafting is nice--but that is also something that was a common part of properly-modded FO games before. Technically, it really isn't all that new and as usual is another example of Bethesda depending on the work of fans, but it's still great to see it arrive in the vanilla release. I mean, the end result is what matters, right?
I personally think the game looks fine for what it is, but I also understand the need and desire for higher-res textures and visuals in a game released in 2015. Yes, I think it looks good, but this isn't "crippled" like DAI was last year, with PS3/360 requirements, and yet those managed to look a bit better. Granted, not the same type of open world.
In the end, I'm not disappointed because it's what one comes to expect and mostly what people want from a FO game. But, you should still some desire and effort from the Devs to move quality along and work on old and long-broken mechanics. Then again, I imagine that the same disappointed people would cry about "too much change! why fix what isn't broken!" on the other side....as would those that really just wanted more of the same. So, in the end, it's probably least problematic to just crank out the same game.
Hell, look at Madden and COD. Which is probably the core of the criticism--FO and TES players don't want this company to follow the EA model, even though it has been happening for some time.