In the first eight seasons, Cosmo was simply The Ditz and a bit of a CloudCuckooLander, sometimes dim but still relatively normal, and perhaps strangest of all, he and Wanda seemed to be on an equal level of air-headed and careless intelligence. Wanda was portrayed as being dimmer than Cosmo, believe it or not, then at some point, they shared an equal amount of intelligence. But in these seasons, Cosmo became simply just The Ditz as opposed to being both that and an average Cloudcuckoolander, and Wanda became simply the wiser one who chastised Timmy and Cosmo whenever they did something wrong, but then all her jokes became nagging.
Speaking of which, Wanda seems to spend most of her time nagging.
They both seem to lose their wands a lot in these seasons, particularly in season 10 and depend on Timmy and Chloe to solve the problems on their own.
While Wanda's voice is still fine, Cosmo's voice is now high pitched and it just sounds terrible and grating, an issue that still persists as of the recent Fairly Odder reboot.
As if BQ#3 wasn't bad enough. Cosmo kept both his and Wanda's wands in his pocket the whole time in the episode "Which is Wish".
They are still there for Timmy and later Chloe when he and she need them.