The only notable improvements are the extended interviews and some tweaking to the picture and sound quality. I own the original set as well and it's comparable, so I wouldn't be in any rush to get this if you already have the first set.
And another feature is John Cleese commentary for all twelve episodes. Not something that was included with the previous Fawlty Towers set. I know some people don't like listening to commentary, but I'm having a blast reliving those twelve episodes.
That 500 minutes is derived from the all the "bonus features" on the 2001 release. They include interviews with Cleese, Scales, Sachs, and a documentary called 'Torquay tourist guide'. Those extras are on the third disc of the 2001 release. Let us not forget that the previous release had director's commentary of John Howard Davies and Bob Spiers (first and second series, respectively). I have not listened to the 12 Cleese commentaries but if you're one of his fans I suggest you purchase this set. Currently it's very reasonably priced at around twenty five US dollars. It surely will go up in price in the long term given the recent release date.
Update: The Cleese commentaries are hilarious! He nitpicks every little mistake made, minor detail that stood out and even critiques his own actions, saying how he could have improved the "funny" in his dialogue deliverance 30+ years ago!! Well worth it to upgrade given the reasonable price.
John Cleese's commentary during the episodes often explain his creative process! As an aspiring writer, I am grateful for his insight. It is very generous of him, really. I think the directors' comments during the episodes might also be helpful to any aspiring director. I am a huge fan of Fawlty Towers and John Cleese!