My daughter watches this show almost every day. It's wonderful! She loves all of the cute characters and it's educational. It's catchy with the cheerful, bright colored characters and teaches useful knowledge about the world to help prepare them for school by teaching letters and numbers. It shows diversity and promotes harmony without getting political about anything. There's songs when appropriate and the voices in the show are great at retaining a toddler's attention even when they're not singing.
As someone else pointed out, it does not teach foreign languages, which as a foreign language teacher, I think is a good thing about this show. Toddlers should focus on learning their first language or the languages spoken by their caretakers and family before learning an unfamiliar language. They have enough to learn in those first few years. It is unlikely for kids to remember each of the random foreign words taught each day when they are never used by anyone else in real life and are not said in subsequent shows. It also gives kids a false sense of what learning a foreign language is like possibly making it difficult to accept in middle school when they need to learn how to form sentences and remember words in order to communicate on any level besides saying hello. Sesame Street shows them many everyday aspects of the world they actually live in.