The thing I am going to miss is... DRE, Ceilio's, Scramble and Castaway. Those cards have become staples in nearly every deck. DRE needs no excuse. 2 Energy is to good to pass up.Espesially with Gallade, who KO everything anyway. Scramble is great. Saying good-bye to it will be hard. No more amazing comebacks when your running low on Energy when you need it most. No more advantages of letting a starter get KOed. It will be sad. Castaway is a great card. Lapras GE can't replace it, as he needs Energy. Castaway was a great card that lets you get an Energy, Supporter and Tool from your deck. Not many decks could afford to not play at least one of it. Ceilio's has become pretty much the best card in the format. Yes, better then Gallade, Magmortar... think about it. What decks DON'T run it? Bebe's can replace it, but Ceilio's will always be dear to our hearts. It was always such a good card.
Another card that will really get hurt by it is Blissey. No Boost = no quick 30 extra damage out of nowhere. No Scramble means that huge comebacks doing like 100+ won't happen anymore. The rotation of Special Energy will bring a new era to Pokemon. We've become so reliant on them that I guess PUI wanted to see what we'd do without them. So ends my conclusion on what will be missed greatly by many players. I hope that we get reprints of all these cards, but I guess not. However, there are some new things that will make amends for what was lost:
First off, I have to say Rhyperior Lv.X is broken. A possible 250 for 0 Energy is wonderful. Even without Mantine d in the format, Rhyperior Lv.X will still be insanely broken. Lose your deck, while doing 0-250 a turn, then eventually put all Energy back into your deck for three Energy! Magnezone Lv.X will definatly see some play. Auto-Paralyzis and 80 for two Energy is just too good to pass up. Mewtwo Lv.X gains imunity to Basics, which can bve HUGE in the format it's going to be in, as I'll explain below. Plus, it OHKOs Gallade = pwnage. Groudon is insanely fast, and will be the next T2 deck, or Kyogre will be very good too. Rayquaza will be great, but only being able to have one in play to use it's Power is bad.
Overall, the new set will introduce us to new cards and strategies that will change the way we play. Cheap Energy costs means that even without Special Energy, Pokemon will continue to be a fast and enjoyable game. However, some of the staples we have now will become rotated, and so we are forced to play alternatives or do without. This is what I think the format will be like next format.