Wrong. The key to every great game is submersion. That in no way applies to every little thing being "thought out". You're nit picking. It's not that distracting. I've never been in a battle so intense that the level up notification got me killed.
I did counter your arguments. You said you wanted it, 10 others said they liked the new system as much as the old one or better. Hence the design choice won out among 90% of the gamers while 10% complained about it. Seems like just another day in forumsville.
The screen pop-up bothered me, just not enough for me to voice my opinion of it.
To me, I don't find it that distracting. In fact, it's pretty thrilling to see and hear it go off in the middle of a fire fight. At any rate, it seems like you really just don't like the audio going off and interrupting other audio. I'm sure that once the GECK drops early next year, if you ask nicely, someone can replace it with a much simpler sound - if you don't know how to mod it. If you do, that should be easy.
I like being able to take that perk i've been waiting for right in the middle of a skirmish instead of having to wait till after the fight has ended.
It's awesome IMO. It saved me a couple of times during a battle bc it restores 100% health when you level up.
I much prefer FO4's way of doing it!
All the reasons as to why have already been stated by others.