Some say it's the new designs looking lazy and others say the formula has become too familiar -I have a complaint not many people tend to address and what I believe is why some people cannot stand the newer Pokemon designs. For me, the biggest problem with Pokemon is the completely different art style it has now from then.
I don't like how his style is is less detailed and more... round nowadays XD I miss the old Akira Toriyama inspired look the first 2 gens had. That's my biggest complaint about the series over the years. The new art makes some Pokemon like Bewear look flat because they rely on the shading to make it look 3D but it doesn't x.x
Maybe I'm just crazy but that's my weird complaint. Here's a few more comparisons:
Gen 1: 
Gen 2:
Gen 3:
Gen 4:
Gen 5:
Gen 6:
Gen 7:
