The park itself has just changed a lot. Bubbleworks is obviously something I miss, but it's not the deal breaker.
Chessington used to be very thematic; each area had clear borders and very indicative theming. Now, however, it's just a bunch of rotting bits of machinery strewn haphazardly across the landscape. Wild Asia is a decently themed area, not to fond of all the grey, but everywhere else is just a bit messy.
Mostly, however, I just miss all the characters. For me, and I in no way expect this to be unanimous, the best thing about theme parks were the ride mascots. Not the shoehorned sponsor ones ala gruffalo, but the classics like Professor Burp, Abdab, Peg Leg Pete. I'm having a hard time expressing what made the experience so special, but I really don't care about what a ride does. I just want to be lulled away to a land of make believe.
The only thing that comes close these days, and I feel a bit sick saying this, is Disneyland Paris. It's slowly losing its charm, but it's the closest I've got to experiencing what old Chessington was like.
Oh, and I miss the monorail. I hadn't been been on it since I was old enough to realise there were better things to do with my time. Now that it's gone, however, I pine for an aerial look at the park.
Oh, and the musical weather vane...
...and the chocolate castle...
...and that beefeater that had a suction mouth. Was he a bin? Is something still there in his place?
Also, might I add, I'm actually not big on thrill rides. My top 5 rides of all time are:
1. Bubbleworks (r.i.p)
2. Terror Tomb (r.i.p)
3. River Caves at Blackpool
4. Pirates of the Caribbean (pre Jack sparrow obvz)
and 5. Phantom manor.
As you can see, I just like to sit down, nice and slow, and bob past some scenes. I can get my thrills on the M25 when some juggernaut starts veering into my lane.