I've always thought it comes down to your own preference. Do you want rides or a park experience?
I don't go to a park for the rides primarily, I go for the day out and if I grab some coasters and rides too, then it's a bonus
Seriously, I'm not one for re-rides and I like to spend a lot of time just moping about and chilling. So a park has to offer that other side.
Thorpe just doesn't. I thinks it's got some fine rides and it's not an unattractive park, it's just dull. If the park is empty, then after about two hours I'm done. I've been on all the rides and there's nowhere I just want to "hang about". I also find something in the atmosphere that's a little oppressive and I think it has to be the clientèle.
Chessington I find really dull ride-wise, but I always have a great day there - it's almost the inverse of Thorpe for me. Drayton, Flamingoland, et al I find I can enjoy a day there, but it's never outstanding - just "yes, that was good".
Alton though I just love the time I spend there and when I do want to ride, there are some rides I adore (obviously Nemesis and Oblivion, but I also love Hex and the Rapids). I like that you're constantly on the move and roaming from one area to another and it's completely different. You never find yourself in the same place twice. Alton was buzzing yesterday too. It's quite an uplifting atmosphere from the crowds you just don't get at Thorpe. It helps they have the characters going around creating a buzz, but at the end of the day, with everyone tired and waiting a long time for the monorail, is still smiling and generally pretty happy. You see people enjoying themselves and it's catching. I know what people mean about Alton losing it's magic, but there is still something uniquely uplifting about the park.
On the subject of losing it's magic, I think the main reason is simply because they continue to rely on the past foundations, but never improving or creating anything quite so good themselves (Merlin that is).
I love the walk from just before the Rapids/Katanga Canyon to Gloomy wood. It's such a well designed and presented area of theme park, it's brilliant. Hex uses the Towers excellently, and the Towers themselves are such a superb centre-piece. Forbidden Valley is still stark and imposing, but... All these places aren't the same as they used to be. Merlin have "sanitised" the areas in a corporate manner. They're still good underneath, but they're just not the same. New additions (like Dark Forest) are "adequate", but they're just not quite right. The effort is the least they could get away with, it just doesn't show flair or shine as it used to. It's all relying on the good old framework, but deprecating it over time rather than enhancing it.