>>123119241. I thought the region was really boring, I played through it a couple of times, and can't remember anything about it, whether it be regions, gym leaders, elite 4, etc. Nothing felt like it had any personality.
2. The evil teams were freaking stupid. I don't think this is debatable.
3. The legendary pokemon played a central to the point of the plot, but they felt really bland and uninspired to me, like blobby shapes covered in glowing lines. I'm not saying the legendaries that came after were good either (cough, chough DPPt), but coming down from legendaries like mewtwo, ho-oh, and lugia was a letdown.
4. I thought double battles were lame compared to single battles, and it seemed that they spent a significant portion of the pokedex adding in pokemon made specifically for doubles which were useless in the other 99% of the game.
On the other hand, they did some things very well.
1. Best general pokemon designs since gen 1
2. Natures and abilities are maybe the most important part of what keeps the competitive scene alive
3. Contests were great fun
Gen 3 added so much to the pokemon franchise that are instrumental to its success, and I think it may have been the most important generation for the series thus far. Despite this, it just wasn't that fun to play! Gen 4 and Gen 5 took the sweeping changes that gen 3 brought in, and actually made it fun again (Gen 4 for the competitive battling, and Gen 5 for the actual gameplay itself).