You're talking about the series from the perspective of those watching it weekly in Japan, and not really taking into account that most people hear probably watched DBZ first, or at least were introduced to DBZ first (myself included). Now, of course, that might not be much of an argument since I'm sure that you could at least agree that FUNimation's dub of DBZ is clearly distinguishable from either their interpretation of DB or the original Japanese version of DB, but the thing is, I have watched quite a lot of the original DB and DBZ (though, with subtitles, of course), and I still feel that there are many differences between the 2 series. I even felt that way when reading the manga, in how different the beginning was from the later parts of the story.
I never said that Raditz's arrival was the more "serious" turning point of DB, myself. But I also wouldn't say that it was the 22nd Budokai was, either. It was when Piccolo stepped into the story that things took a more serious turn, but evne then there were still plenty of parts that had that classic DB sense of humor, such as Pilaf serving under Piccolo and serving as a minor comic relief of sorts for a brief time, and there was also Yajirobe, who was also another clear comic relief character that you would expect to come out of early DB.
In terms of Tenshinhan being threatening, I don't see it the same way as you do, since he was on the same general level of skill and power as Goku was. When I said serious threat, I meant enemies who far outpowered Goku, to the point in which even after becoming stronger, he still needed help from others to defeat the opponent. At the beginning of DBZ, Master Roshi implies that Goku has gotten stronger over the past 5 years, and how it surprised hime because he expected him to be much weaker, if anything, being that he probably didn't expect Goku to really keep up with his training. Yet, even then he needed the help of Piccolo to defeat Raditz, and even then they just barely managed to do it by sacrificing Goku's own life (the 1st time that Goku actually died at all, which made Raditz more than even a thread, if anything). And of course, as far as Vegeta goes, he was far stronger than Goku was, to the point that Goku even admitted that he was outclassed by him, even after all of Kaio's training (wecept for when he used Kaio-Ken x3 or higher, which gave him an edge, but he couldn't even keep it up for nearly long enough). In theend it took the combined efforts of him, Gohan, Krillin, and even Yajirobe to defeat Vegeta, and even then they got really lucky with Gohan's involunatry transformation catching Vegeta off-guard at the last second.
Of course, when you think about it, didn't Vegeta purposely instruct Nappa to be careful with the amount of destruction that he caused, so that they wouldn't accidentally destory any dragon balls, and also so that they wouldn't hurt the price they could geto n the planet by damaging it too much? In fact, Nappa was holding back most of the time until Goku arrived (in which case he didn't even have a chance to create any carnage, after being completely dominated in battle). And as for Vegeta, Goku and company were all there to keep his focus on trying to kill them (and when he did actually try to destory the Earth, Goku was just barely able to stop him by going Kaio-Ken x4). In contrast, Piccolo didn't have any real strong opposition to stop him until Goku showed up for a re-match with him, after having had a huge power-up.
That still doesn't explain how those "first 3 arcs" are so drastically different from later arcs. And even the later arcs of DB (referring to the anime, here, for obvious reasons) have significant differences from DBZ arcs, as I've pointed out before.
Also, in early DB (at least from what I read in the manga), Goku sends that whole Rabbit team to the moon on his power-pole. Is there even a gag in DBZ that is even close to as ridiculously over-the-top as that? I mean, I know its just one minor thing scene on a single page, but seriously, small moments like that definte huge differences to me.
Anyways, I'll just leave it off in that I think whether DB and DBZ (or as far as the manga goes, the first 16 volumes of DB compared to the rest) are very different or not is something that's purely up to how one interprets both series. I personally interpretted each very differently, and I always will see them as being very different, though they will always have that 1 core element that makes them both DB, in the end. I'd go into other details, like how I feel that Goku is more naive up until his teenage years, as opposed to his adult version, which is still child-like but also noteably more mature in certain ways, but I'm way too tired of typing now, so perhaps I'll save that for another day.
