I remember playing DnD 2nd back in the good old days. Ahh, Mages that rolled 1d4 / level for HP and which cast Fireball to hit an enormous area for 1d6 / level damage... good times. And by that I mean what the hell were you idiots thinking?!? The funny thing is that Fireball was hideously overpowered back then because when cast against a reasonable encounter it would often take off 70% of the HP the enemies have in total but now it isn't nearly so problematic because players and monsters have scaled so much. Instead of Mages having 2.5 HP / level the standard these days is 8 HP / level and suddenly you can take a fireball and expect everyone in the group to be hurt but still very much in the fight. Other things have changed too since Rogues actually do real damage, all brawler types have access to much higher hit and damage bonuses from feats and class features and magic items that boost up stats are much more widely available. Of course along with these buffs came monsters of increased power - you can't just use a 2nd edition monster against a Pathfinder party because it will die in 1 round and accomplish nothing.
This creates some weird scaling issues though. It used to be that you had a choice between casting fireball (which very nearly wins the fight on its own) or summon monster 3 (which summons a really crappy monster with 3 hit dice). After all these new editions and changes though fireball is still the same as it ever was but summoned monsters got massive stat and hit dice boosts and incredible specials like the Bison which got Trample. As far as I can tell from the rules the way Trample works is the creature walks over as many enemies as it wants in the turn doing 19 damage to each of them and knocking them down - they can take half damage with a really difficult save. This is more powerful than Fireball but the Bison is going to stick around for 7 turns and do this disgusting AOE attack every round. Trample needs a big nerf.
This is the same sort of issue 3rd edition ran into with Polymorph. 3rd edition monsters were so much more powerful than previous editions that the ability to turn into whichever one you want became completely insane and let casters turn into Stone Giants / Dire Bears or whatever happened to have the appropriate mix of necessary stats and destroy people. This also gave a huge power boost to anyone who happened to have more Monster Manuals around since they could find more interesting things to turn into. It also meant that any time a new monster was published there were really nasty design constraints because players were so happy to turn into anything that had high physical stats or AC or other extraordinary powers. Of course this might be a minor concern because once you had the option to turn into a Stone Giant a new entry would have to be truly stupendous to be even more broken. Pathfinder fixed this problem by making all the polymorph spells give fixed stat and AC bonuses and their system works pretty well I think but they really missed the progression problems with animal summons.
I will say though it is really nice to have summons be a decent option. Having casters always default to massive AOE damage to attack isn't very interesting compared to choosing an appropriate animal to summon for the moment. If I want to lock one target down I summon a Crocodile and it can grapple and Death Roll! the target. If I want pure beatdown I summon a Cheetah and my lower level options include Giant Toads and Eagles depending on what sort of problem I am having. This means that characters and the enemies are involved in melee combat dealing with grappling, tripping and positioning instead of just waiting for the massive AOE to land. I like that style of combat a lot more, though I will admit it was outrageous fun to Fireball the enemies to oblivion in the old days. It wasn't as great when we got Fireballed in return though, which is why the new system is a lot better.