Could you say what specifically you thought got worse about it as it went on?
Level design goes down the pan and the new mechanics are revealed for their half baked application to the gameplay.
Basically Sonic boast these new tricks like parkour which contrary to its naming only really allows him to slowly and stiffly jog a few steps up a wall which could have been neat if there any actual speed and flow to it. They bring back the wisps/colour powers but in a way that barely compliments the level design and did include a heap of forced gyro stuff until Sega patched it because
we had to tell them how bad asteroid wisp was.
Level design is caught between a number of spots, it wants to be both Sonic and to an extent 3D Mario, it succeeds at neither with a stilted sense of speed to accommodate the attempt at a stronger platforming focus. 3D levels often make use of cylinders that present a path on both the topside and underside but they come across as so open half the time that there's not really much to contend with. So the game spends much more time in 2D where its parkour and speed focuses are hindered further.
The ice mechanics of world 4 alone would be enough to dismiss it and the level design appears to have run out of tricks at this point with nothing much new to show. It's kind of hard to really explain Lost World, things just start getting noticeably worse at a fast pace and the game is all over the place.
Judging by peoples reactions to it including other Sonic fans the fact that I managed to wrangle some enjoyment out of it is quite the feat, another case where I felt that Sonic Team had their hearts in the right place and then proceeded to fuck it all up because that's kind of their thing.
Give me your copy of Snoic Lost World, because I was bored 5 minutes in and it never grabbed my attention.
The damning thing here is I'm trying to think of a reason as to why it grabbed my attention in the early goings and I'm left at a blank outside of the visual style and music striking a chord, I thought there was potential in the new mechanics!