I'm not really sure if this goes under "Unpopular Opinions" but it's worth a shot. I really don't understand why Gen 5 gets so much hate for it's Pokemon designs. There have been several Pokemon designs that been outright disgusting and bland, such as Muk and Voltorb respectively. Despite all of this Gen 5 seems to be on receiving end of all the hate despite having several Pokemon with good designs.
For me, it's got something to do with variety and diversity, and something to do with execution of concepts.
To begin with the latter, Garbodor and the Vanillite line are the most commonly critiqued Pokémon of Gen V. As you say, their concepts aren't too far removed from what has been pulled off successfully earlier, but I think the execution is flawed somewhat. Garbodor got unlucky with its face and its pose. It looked plain stupid, and that randomly waving arm in the concept art and ingame sprites really hurt its image. It ended up much more dumb-looking than it needed to, leading to many people criticizing its design. I was among them for a long time, but I've later come to realize that it's its
looks I can't stand. With only a few minor tweaks to its appearance - its goofy face and its twitchy pose, mostly - Garbodor can look surprisingly good.
Vanillite's line is a little flawed in another direction: They look too much like ice cream with faces and too little like anything else. Other Pokémon get this sort of complaints too. Sunflora is an early example, Glalie got some flak too. The "object with a face" look can work fine if the design doesn't resemble a specific object too much, but if it does it goes straight into the uncanny valley. Same goes for some of the lazier animal-based designs. You never saw many fans of Goldeen and Stantler around, and Ducklett was ridiculed for the same reasons.
As for the "variety and diversity" bit, that's my biggest gripe with Gen Vs designs. Many of them felt like complete rehashes of older Pokémon designs, only for the sake of creating more new content. To take one example: for all intents and purposes, Tympole traces Poliwag's concept. Pure Water type blue tadpole that evolves twice, the first time at level 25, found in shallow ponds. That sort of specific properties works fine once, but don't try to sell it to me twice and pretend it's something new. I'm fine with some concepts being rehashed over and over again throughout the generations (the early birds, bugs and rodents), but when so many of the new Pokémon completely emulate old ones, I'm less impressed. Roggenrola and Geodude. Timburr and Machop. Drilbur and Sandshrew. Dwebble and Paras. Throh&Sawk and the Hitmons. Audino and Chansey. Munna and Drowzee. Practically half of the Unova Pokédex emulates concepts we've seen earlier, often combining the same design basis with the same type (Normal type nurse, Psychic tapir, Fighting strongman, etc). These are interesting concepts when used once, but not twice. With the exclusion of Gen I-IV Pokémon from BW, it felt like Game Freak didn't even want to acknowledge the older Pokémon, instead filling the exact same roles with new designs that so blatantly resembled the old ones. And then they pushed it as something new and refreshing we hadn't seen before. No wonder players were upset.
There's also the small case of pushing the same design too many times. Elemental monkeys. Do I need to say any more? Great and original concept, but making three sets of functionally identical Pokémon got old quite fast. Especially since they insisted on always, without exception, bringing up the entire trio every time. The legendary genies were criticized for the same thing, they were for practical purposes the same design used thrice.
It has to be acknowledged, though, that Gen V does have a great deal of cool, new and original designs. The latter half of the Unova Pokédex is brim-filled with stuff we clearly never saw before, Pokémon based on new and interesting concepts, most of them really well executed. It's as if Game Freak felt obliged to find replacements for a lot of older Pokémon when designing Gen V, and when they were done they were free to let the creative juices flow. It's too bad that the copy-and-paste-job of the first half of the 'dex overshadowed the top-notch creativity displayed in the latter. Normally, the practise of design reuse is limited enough that you can see past it (you put the early bugs and rodents behind you quickly enough), but in Unova it was too extensive in-your-face.