I think Sinnoh simple because this is the first time that Ash's pokemon learned moves that were not either of their own type, or simply Normal type, as their main battling moves.
Pikachu, Charizard, Noctowl and Snorlax are acceptions to this, I know, but for the most part it stands:
Bulbasaur: Razor Leaf, Vine Whip, Leech Seed, SolarBeam (all grass)
Squirtle: Hydro Pump, Water Gun, Rapid Spin, Skull Bash (two water, two normal)
Bayleef: Razor Leaf, Vine Whip, Body Slam, Sweet Scent (two grass, two normal)
Muk: Poison Gas, Sludge Bomb, Body Slam (two poison, one normal)
The list goes on.
And Ash is repeating this in Unova. Alright, Krookodile hasn't fallen into this trap, each move he knows is a differnt type, but others have.
Oshawott now seems to know only water moves, and Pignite only fire. Snivy's three damaging moves are grass type, with Attract for backup. Unfezant knows three flying moves and one normal. Leavanny now appears to be limited to two grass moves and String Shot only (had it used Bug Bite since evolving from Sewaddle?)
Scraggy sort of gets away with it, and Palpitoad and Boldore are better than the rest in terms of move variation. Palpitoad just needs to get a new one.
However, when looking at Ash's Sinnoh team, you do see variation (in most):
Staraptor: Quick Attack, Aerial Ace, Brave Bird, Close Combat (one normal, one fighting, two flying)
Infernape: Flamethrower, Flare Blitz, Mach Punch, Dig (two fire, one fighting, one ground)
Gliscor: X-Scissor, Giga Impact, Fire Fang, Stone Edge (one bug, one normal, one fire and one rock)
Buizel: Water Gun, Aqua Jet, Ice Punch, Sonic Boom (two water, one ice, one normal)
Gible: Draco Meteor, Dragon Pulse, Rock Smash, Dig (two dragon, one fighting, one ground)
Then Torterra kind of let the side down a bit with three grass move and one normal, but hey at least he had a healing move (Ash just sucked at using it at the right time, and Rock Climb was usually a let down)
Ash's Pokemon in Sinnoh were frequently able to pull off surprise moves on their foes, like Infernape using the super-effective Dig on Paul's Electivire, or Gliscor using Giga Impact on Drapion before flipping through the air to get to safety.
Ash pulled off well planned stategies in Sinnoh. Admittedly this began in Hoenn with thing like Thunder Armor for Pikachu and Swellow, but only in Sinnoh was this used effectively, with the creation, and sometimes repeated use of Counter Shield, Ice Aqua Jet, Underground Flare Blitz and the Volt Tackle/Iron Tail combo that brought down Latios.
This stuff doesn't seem to show up in Unova anymore, and Ash's team seems to be lacking because of it.
For anyone interested, by friend code is 5257-9947-2315. If anyone wants to add me, remember to PM me so I can add you too.
My friend safari contains the ground type pokemon Phanpy, Marowak and Palpitoad.
Current Y team consists of Delphox, Pidgeot, Raichu, Simipour, Charizard and Gogoat.
In training: Wartortle, Bulbasaur, Chespin, Froakie, Combusken, Pangoro.