Gen III might be one of those "love it or hate it" things.
Whether or not it is, it's still bad in comparison t' other PokeRPGs.
... Not that it matters, usin' popular opinion for or against any argument is generally foolhardy. 'Least it is for a matter like a vidya gaem.
Whatever floats your Dive slave.
The vast majority of my in real life friends (and other people that I don't talk to anymore) quite enjoyed Gen III.
Lets see. When you start, you're moving to the town, giving the protagonist reasons for starting his journey. This feels more natural compared to Red/blue where you suddenly decide to do stuff for some reason. Next, one of the gym leaders was his dad. This wasn't much of a big deal, but it did give a reason to what the heck was going on in the guy's life. Furthermore, your rival isn't a douche. It's more of a friendly rivalry this time around.
Then we got the villains. Unlike Rocket, these weren't just some gangsters without a clear goal that you just sorta run into. These were evil men that wanted to destroy the world(or just dramatically change it). So we got the traditional "boy who saves the world" scenario, common in every RPG. Additionally, this wasn't just one group striving to reach it's evil goals, we had TWO of these guys. This means there's two rival groups each trying to grind the other one down, and the protagonist encounters them as they fight for supremacy.
Then we got the theme and lore. RSE feature an overall weather theme, including elements and such. This means the guys on the cover are actually important to the plot. There's a myth that the water and fire/land godlike pokemon fought and eventually had to go into a slumber. If one could use their power, sh*t would happen. And once again, the villains come into play. This is an important occasion where the villains and the legendary pokemon come together. In other games, these 2 were separate things. Team Magma and Aqua want to use a mythical power to change the world. And then they actually go ahead and show this when, for example, Kyogre is awakened. It starts to rain everywhere. This was completely unexpected, and other games never had such a story-related impact.
Some minor RPG mechanics would be the introduction of Pokemon abilities and nature. This is a very important part of these types of games. Uniqueness to party members. For example, in FFVII you could virtually teach everyone everything. They had no abilities unique to each other. There was also the introduction of Pokemon contest, or whatever they're called. Minigames are also a long standby in RPG's, and this one was a quite elaborate. Then there's double battles.
Then there's personal things like the Pokemon and the design. I overall enjoyed them. Lots of water too.