I always had problems with AF S1 and 2 Ben and my outward reasoning was because he felt too generic and boring, but I knew that there was something else to it but couldn't quite place my finger on it. Then when rewatching Alien Force there's a scene when Gwen is trying to convince Ben to ask out Julie, in which she says:
"You're weird, funny, sensitive, well mannered."
This quote by itself made everything click in my mind. The reason why AF Ben doesn't fully work is because the writers were going for a completely different kind of protagonist than the one whom we saw in the original series.
Is Ben weird? To a degree I guess but I don't see him as being overly weird, if he was going on a date I see him more as the kind of guy who tries too hard to impress the girl
Is he funny? Ok maybe.
Sensitive? Err...I don't think that we're talking about the same person here. Original Ben was anything but sensitive a lot of the time, with personal feelings not usually being the first thing on his mind (although he still cared deeply for others close to him). Obviously 5 years quite a bit of time to be humbled by life experiences but I don't think that Ben would lose his headstrong nature completely.
Well mannered? Again, people do change and Ben obviously wouldn't be brattish anymore and would be more careful about what comes out of his mouth. But his rebellious streak and slight arrogance are some of his defining traits which it just feels weird to see disappear.
So from that I can conclude that they were trying to make AF Ben an introspective, slightly awkward, and serious teenage protagonist. In fact with this in mind, it is really evident when you see Ben on screen in AF that they were going for this kind of character. I'm not saying that it's bad to have that kind of protagonist in a story at all (AF Ben is probably what I would be like if I were in his position lol. ). But it isn't who Ben is - in fact it's very much the opposite of who Ben should be. Ben is a headstrong, extroverted, fun loving main character with a slight touch of arrogance, but when it comes down to it he knows where his responsibilities lie just like Spider-Man. Ultimate Alien and Omniverse Ben came far closer to this but what the writers did not fully understand is that cockiness and immaturity are not one and the same. UA and OV Ben both had some great moments where the balance was found, but more often than not they ended up being skewed one way or another: UA swayed massively from Ben being obnoxious one episode to being overly serious in the next, OV Ben was usually aloof and also obnoxious to the point of being irritating.
This is meant to be Ben's story and it's a shame that the titular character himself goes through so many regressions and changes to his personality throughout. Even the original series wasn't perfect when it came to this with Ben suffering a bit of regression and developing an slightly annoying egotistical nature in OS season 3 despite having learned his lessons. But the original definitely nailed who he is as a character the best, and I believe that teenage Ben is perfectly capable of being mature at the same time as being headstrong, rebellious and intriguing as a character.