Changes I would make are:
1. Ensure the 2 dance stage starts early enough in the series to ensure all the finalists have danced all the dances prior to the final. That way the final can be about recapping favourite dances, judges choice etc and focussing on learning the one new routine in the show dance.
2. Get rid of the props, gimmicks and new rules on breaking hold etc. The show is about learning to dance, and managing that 1 minute 30 second routine. Not faffing around for half of it with people bursting out of shells, puddings etc etc.
3. Save the showdance to be the final dance - not for any voting reasons, but because the showdance should be about the culmination of everything learnt over the course of the show and the final highlight of the series....but let all 3 couples dance it. Why not have the whole final be between the final 3.
4. Go back to leaving the showdance unscored by the judges (sort of linked to point 3).
5. Stop using other dance styles for the showdance. It used to be about showcasing the styles of dance done over the series with a few lifts thrown in for effect. Now they seem to use disco, hip hop, street, gymnastics and whatever else, and it shows that the pros too are not necessarily experts in that style of dance. (think Bacofoilgate)
6. Don't get celebrities who are just there to mess around. At least most of the duffers prior to series 7 were trying hard to learn to dance and wanted to learn to dance. They were bad but didn't resort to silly [unfunny] comedy routines. Ann Widdecombe was a step too far given her refusal to learn certain dances/steps and the endless promotion of her afterwards was tedious. I'd probably put Russell Grant in the same category - whilst he wanted to learn and was trying, (sort of trying), he did far too much camping up and messing around. Again the promotion of him is tedious.
7. Get rid of those flippin' VTs! I just want to see them in the training rooms with a bit of chatting about how the week is going. Not trying assualt courses, abseiling off buildings, messing around on fairground rides and generally hamming it up. It also wastes precious rehearsal time!
8. Stop pushing the line that the latin dances are just about sex. They really aren't.
9. More pro dances showcasing ballroom and latin, choreographed by the pros (as opposed to external choreographers). Not random other dance styles.
Ooops despite my criticisms I still watch SCD with all it's faults every autumn!