The ones from RSC were easy, because they're pokemon were repetitive and predictable.
GSC was a bit too easy, but mainly because of the low growth rate of most pokemon in GSC compared to others (most of them were only LV35-40 by the time you reached the E4). RSC also had a low growth rate (not as low), but then, the levels used in the E4 were much more challenging.
RBY was just insane with growth rates.
In this generation, the levels were above average and so was the growth rate. By the time you finished Victory Road, you would be around LV48-55, which was just enough when factoring in levels. The variety, however, was much less predictable, rather than thinking in RSE that Phoebe's going to send out a Ghost-type anyway, so I'll send out an Absol to block their Shadow Balls and kill them with Crunch and etc., in this generation (particularly Flint), there was too much variety to form a legitimate counter to most of the E4's pokemon (in RSE, a Swampert (with Surf, Ice Beam, Earthquake) can kill everything in Ruby and Sapphire).
Pokemon worthy of note in DP were:
Quagsire- annoying, unless you kill it quick (Torterra users will find it to be a pushover)
Lopunny- first-time lucario users may find that some attacks may not OHKO and Lopunny can deal massive damage with Fire Punch. Charm is also pretty annoying
Infernape- Speed kills alot with Flare Blitz and Earthquake
Alakazam- too damn fast to outspeed. psychic, focus blast and energy ball forming core moveset that few can counter
Bronzong- massive defences. enough said
Garchomp- able to OHKO Abomasnow unless it uses Earthquake.