2.5D is kind of like the hipster of game styles. It's lazy 3D, lazy 2D, and its games are always clunky, short, and pretentious. To this day, there is only one 2.5D game that didn't commit one of the sins that I hate the most about 2.5D games. Only one game in that style that I actually like. LittleBig Planet. The 2.5D model was done really well in that game, and it's the only game I've ever seen, let alone played, that was any good. Most of what I'm a bout to go into is exclusively targeting platformers, but I'll throw in other genres to make points where possible.
There are quite a few reasons I hate 2.5D. The first is that almost all of the games are an eyesore. The foreground (anything that can cross over the top of the player), background (anything that the player crosses in front of), and mid ground (the player's area and solid stage) can some times be hard to distinguish from one another, causing death, confusion, frustration, etc. It's worse when objects cross depths, and become part of the stage.
The next big one is the temptation a lot of these games fall victim to. There seems to be this desire for developers to over-blow the background, and stuck way too much clashing, distracting junk back there.
Third gripe is the tendency for a lot of these games to have the character inhabit only a tiny space on the screen at all times. This works well for a lot of 2D games, but with 2.5D and the habit of creating backgrounds that are far too cluttered and distracting, it becomes rather bothersome to me to be looking at the lower 1/6th of the screen the entire time while the rest of the 5/6ths are occupied with way too much junk.
Now let's take a look at something other than the background. The physics. These games are almost always floaty, clunky, and awkward. If it's not the background with all the rendering that needs to be done, it's usually just poor physics. Sonic Generations, New Super Mario Bros (all of them), and Kirby 64, are perfect examples of this. In Sonic Generations, Classic Sonic stages, you can really see the game's poor design when you access Super Sonic I was disappointed that he was so much slower in Sonic Generations than Sonic 2 or 3/And Knuckles. I'm not perfectly sure if that's just terrible programming or to hide the game's pop-in. With New Super Mario, or any of the 2.5D Mario games, for that matter, Mario is much more floaty than Marios 1-3, Mario World, Mario Land 1 and 2, and even Mario 64. New Super Mario DS is actually the worst of the bunch. He even seems to run slower. It makes the game feel too easy, which is is probably the point, since Nintendo is kneecapping itself with how easy it's making Mario games. (Oh well, there's still Legend of Zelda...) One of the biggest reasons I didn't like Kirby 64 was due to how slow everything seemed to move.
Now, I did mention awkward. Double Dragon Neon has awkward written all over it. The game looks great, but the way the characters move just looks terrible. Saints Row 4 did a much better job of it.
The biggest problem with 2.5D games is how bland they are. Short, linear, few to no secrets, repetitive, and lacking substance. Those are words I've used to describe most 2.5D games. Especially platformers. My theory is, so much time and effort goes into cluttering up the background that the developers just forget to make the game good. It feels like all that attention to the background is what causes a lot of these games to be so short. It takes do long to decorate the levels that they shorten them to reduce production time. I could be wrong, but that's my theory anyway.
LittleBig Planet avoids all of the problems I listed above. I said Little Big Planet was the only 2.5D game I liked, that's not entirely true. I really liked Super Paper Mario, but I'm not sure that can be called 2.5D, since it does make use of turning the camera.
Either way, I could keep going, but I'll end my rant here. Even though I probably could do a good 2.5D game, I don't think I will. 2.5D is half-assed 3D, and half-assed 2D. I'd much rather do a 2D game than a 2.5D game.