There seems to be a myth in that in the history of Sonic the Hedgehog games, there was a point in which they went from being great games to complete shit, but the truth is that there has always been hits and misses. Sonic 3D Blast is an example of a relatively early miss. It's just not a fun game. I think I may have enjoyed Sonic Spinball more, and I loathe Sonic Spinball. 3D Blast was just a bad idea from the start. An isometric perspective does not lend itself well to the fast traversal that is the hallmark of a Sonic game.
The basis of the game just isn't interesting. You wander around these bland stages looking for enemies to kill, so you can harvest their little animal insides and deliver them to the checkpoint. I guess that sort of makes it a series of fetch quests/escort missions. It is boring. The whole thing looks like developer Traveller's Tales spent more time pre-rendering all these Donkey Kong Country-style sprites and forgot to make a fun game. There karmic punishment seems to have been the curse of having to make like ten Lego games in the past five years.
With the isometric perspective, you have to control your movement in eight directions, which is something that does not work with a Sonic game's fast pace. It is made even worse by the camera limiting your ability to see very far ahead while moving. I guess this ultimately doesn't matter, because you won't be doing any fast traversal or platforming. The small areas don't give you much ability to move very fast, and so the only reason there is any sense of speed is the overly sensitive movement controls that make every level feel like an ice level.
Platforming is something that doesn't work in an isometric perspective, especially one that is not actually 3D. The Landstalker approach to this problem was to have platforming, but it was bad and frustrating. The 3D Blast approach was to remove any meaningful platforming. Sonic can jump and there are springs are around, but neither really do much for you. What is a Sonic game without the unique and exciting level design, and nothing fun to do? Sonic 3D Blast it seems.