ridiculously good resistances with fantastic typing to boot, solid bulk, amazing speed tier, ridiculous power. I personally don't think latias was broken but there's no denying it was incredibly good. never played with latios but its extra power would really push it over the edge. both of them turn most special attackers into an invitation to drop a specs draco meteor on something. it was also ridiculously easy to run super solid semi-stalls with an amazing defensive core and tons of offensive pressure in the latias era.
here's what happens when salamence comes in, not at all hard to do with its good resists, intimidate and iconic base 100 speed. you have to guess whether it's dd or mix mence. guess wrong? you lose a pokemon to the mix set (sending swampert/hippo/skarm in expecting dd, which still brutalizes them i should add) or get swept by/lose 2 pokemon to the dder (switching scarftar in expecting draco meteor). guess right? you "only" take a brutal amount of damage and still maybe even lose a pokemon since draco meteor is so absurdly powerful, even if you predict it and send in your scarf rachi/ttar/heatran you still get smoked, not to mention a fire blast/earthquake or brick break on the switch and they're toast. base 100 speed was huge, it meant that scarf gon/jira could only hope to tie it and scarf tran stood no chance. it's why dragonite isn't nearly as dangerous, it's less powerful on both sides of the spectrum and much slower, i cannot stress enough how important that last part is. speed kills.
oh and i haven't even mentioned the bulky dd set which turned everyone's favorite revenge killer for it, cb scizor, into setup fodder.
tornadus-t was absurdly fast, beating out scarftar, and had regenerator + u-turn to chip away at its checks (and taunt to prevent them from healing if it so desired). thundurus wasn't quite as fast but beating out lati@s was huge, the nasty plot set was incredibly dangerous, and prankster thunder wave is cancer on everything. landorus was straddling broken territory with the power of its sand force sets in bw1 but sheer force really pushed it over the edge since it was dishing out hits from what was effectively base 170 special attack or something equally ridiculous, when I first saw how much earth power was doing to things I and several others jumped. it took very little support for it to dismantle things; u-turn smashed all its checks in conjunction with pursuit and rock polish swept offense easily, sometimes the two were even used on the same set.