Here's a list of all the Tier 1 and 2 cards we would lose from an XY-on Format, each with a note on what they did.
-N
(Early game draw and late game disruption. Without it, the only AAA Shuffle supporter right now is Shauna)
-Plasma decks
(No duh)
-Empoleon
(Strong water attacker with good draw support. Essentialy being replaced by Ancient Swampert, to an extent.)
-Altaria
(Dragon enhancer. Won't be missed that much, though, since it's only Tier 1 combo was Garchomp, which is rotating as well)
-Celebi EX
(A better form of Shrine of Memories, since it can't be Stadium Countered. Without it, players will have to run several Shrine of Memories in order to run things like Gourgantic Gourgeist and Aggron-EX)
-Blastoise
(Fantastic energy acceleration. Without it, decks like Primal Kyogre-EX will be ever-so-slightly slower, and cards like M. Blastoise-EX will be nearly impossible to play in Tier 1)
-Keldeo EX
(Versatile Switch combo with Float Stone. Without it, Bronzong decks and decks that find trouble against Special Conditions will be harder to use.)
-Dusknoir
(Moves around damage counters. Without it, spread decks like Noivern and Gourgeist XY will have to rely on Meowstic.)
-Landorus EX
(Excessively strong Fighting Type. Without it, straight Fighting Decks only have Lucario-EX in terms of basic Fighting Types that can OHKO.)
-Flygon
(Spread tech. While not great, it was used quite a bit at the 2014 Worlds with Accelgor.)
-Skyla
(Best search Supporter. Without it, decks that require quick access to Trainers will have trouble.)
-Town Map
(Tracks down Prizes. Without it, decks might not be able to make use of 1-of lines of cards.)
-Computer Search
(Broken search card. Everything is slightly harmed from it going away.)
-Victini EX
(Good start-up energy acceleration. Nothing in particular that loses it, but it will be missed.)
-Trubbish
(Fun archetype that revolved around Masquerain and Sigilyph from Plasma Blast, though they all are rotating.)
-Donphan
(Incredible attacker with several comboes. Only major replacement for it is Mienshao, which falls prey to having to re-attach Energy and worry about hand disruption)
-Klinklang
(A cool archetype that paired with cards like Registeel-EX, Cobalion-EX, and Aegislash-EX. Isn't used much anymore, but used to be pretty great.)
-Cobalion EX
(Strong disruption and pierce attacker in Klinklang decks and Bronzong decks. Most of what it does best can be done by other cards nowadays, though.)
-Black Kyurem EX
(A very powerful archetype that comboed with Blastoise for OHKO's turnly.)
-Bicycle
(Quick hand draw item. Only incredible replacement for it is Roller Skates, which requires a coin flip.)
-Colress
(Great late-game draw card. Only suitable replacement right now is Shauna, as mentioned.)
-LaserBank
(Broken damage booster. Without it, Seismitoad-EX has no reliable way to stack quick damage without the use of Grenade Hammer.) why is that a bad thing?
-Dowsing Machine
(Trainer recovery. Is sort-of replaced with Trump Card, but still is really good right now.)
-Exeggcute
(Good Empoleon and Weavile combo. Nothing from XY is really missing it, though, not counting Ultra Ball)
-Flareon/Leafeon
(A strong archetype that focused all around Type advantage. It is really the only super competitive deck that I've ever been able to legitimately play, because of how inexpensive it is.)
-Electrode
(Okay draw support. Was used primarily in decks that need to draw outside of Supporters, because they used tech-supporters like Lysandre, Blacksmith, Flare Grunt, and Fan Club.)
-Mr Mime
(Incredible tech for decks that use bench support. Without it, Pokemon that have low HP will be harder to play against sniping decks.)
-Latias EX
(Attacker primarily used as a Pyroar counter in Plasma and Toolbox. Since Pyroar isn't used much anymore, and won't probably won't be at all once XY5 releases, I don't think this card is that important anyway for the 15-16 season)
-Float Stone
(Good with cards that have high retreat. The only suitable replacement for it, outside of directly switching, is Fairy Garden.)
-Superior Energy Retrieval
(A card used to recover energy in hand attachment acceleration decks. Without it, Sceptile will have to rely on the much weaker Energy Retrieval.)
-Virizion/Genesect EX
(Fantastic archetype. Was quick and powerful, with only a few bad match-ups.)
-Suicune/Sigilyph
(EX counter in many Toolbox and Hit, Run decks. Without it, the only card in the format that is immune to EX's is Pyroar, but only to Basic ones, not Megas.)
-Drifblim
(A strong Plasma and Psychic-weak counter.)
-Jirachi EX
(Absurdly useful Supporter searcher, though it is outclassed a bit by VS Seeker.)
-Haxorus
(Fun Plasma counter and Bronzong mix. Wasn't great, but could hit really high numbers for Stage 2 standards.)
-Iris
(Good damage booster, though it wasn't used often, outside of Lucario and Shaymin-EX.)
-Silver Bangle
(Better Muscle Band for non-EX's. Without it, many of the newer cards like Kingdra PRC and Blaziken FUF are shy of Knock-Outs against 180HP EX's.)
-Victini
(Coin re-flipper. Without it, you'll have to use Trick Coin, which is harder to use because it is a Tool)
-Emboar
(Same reason as Blastoise)
-Outragers
(They've always been good.)
-Seismitoad/Meloetta EX
(Fun archetype that used to be really fun and affordable. Is not that great anymore, though, because of a lack of Level Ball.)
-Mewtwo-EX
(Infamous for being probably the most broken card for it's era, tying with the Hitmonchan and Luxray GL Lvl. X. Without it, the only other Psychic Type that runs on Colorless to hit for weakness is Starmie.)
-Garbodor
(Mind-bogglingly broken Ability Locker. Without it, the Seismitoad-EX archetype is really dead, outside of Ninetales/Silent Lab.) (Kill it now. Just kill it right now. I can't wait until August for it to rotate, so KILL IT NOW.)
-Terrakion
(Great Type advantage revenger. It's one more Fighting Type that will be missed from the Fighting Archetype.)
-Darkrai-EX
(Undenyably useful Dark-Type that was a deck all of it's own. Without it, Yveltal-EX really doesn't have a great free retreat tech anymore.)
-Zoroark
(Fun combo in Dark decks, though the archetype hasn't been used in a while.)
-Cobalion
(An occasional tech in Metal decks to stall for a turn or two, depending on the match-up.)
-Elesa
(An interesting Supporter that is sometimes used in Tool-heavy decks, like Mega decks.)