It's basically a mini-Zapdos that can kind of hurt Rocks with Double Kick, too. Compared to Zapdos, it gets a better Crit rate, can outpace Tauros, Starmie, and even Alakazam, plus it has no Ice weakness so it can go for the crit more easily against Chansey and can afford to stay/switch in against STAB Blizzards from aquatics. So why doesn't it see as much usage as Zapdos? Well, its Defense is garbage, as is its HP, so while it's outpacing Tauros, it isn't taking a hit from it. Snorlax, especially, is a lot for Jolteon to handle, whereas Zapdos often forces the big lug to SelfDestruct. Finally, Jolteon requires 2 unSTABbed coverage moves to hit everything it needs to, which on its own isn't all that bad since Zapdos' 2 moves other than Thunderbolt and Drill Peck are often filler, but it does mean that Jolteon has to predict like mad if it's doing good damage on a turn-by-turn basis while one of those Electric-resisting Pokemon (or Chansey) is still lurking around. Despite its flaws, Jolteon is an excellent mon that kind of acts like a Special Tauros (albeit far more easily walled), and it gets even better in Stadium where Focus Energy actually works and those Physicals that normally destroy Jolteon are nerfed by different Hyper Beam mechanics.
Often considered the best Baton Passer due to its Speed and attacking prowess; for the longest time, GSC's top Phazers were Skarmory and Suicune, and guess what Jolteon just happens to beat? Furthermore, Ground-type Phazers like Rhydon and Steelix take a lot of damage from Hidden Power Water. It also gets Agility, Growth, and even Substitute to pass effectively to its teammates. The popularity of Raikou does take it down a peg, though, meaning it's often easily stopped anyway and therefore probably isn't worth its frailty on any but the fastest of dedicated BP teams. Another fairly unexplored set is the Growth sweeper, where Jolteon doesn't pass Growth but instead uses Rest and its Electric typing to set up a sweep in the face of Electrics. Not as consistent as Vap because it's so much more fragile and is 2HKOed by DE or EQ from Snorlax, but if Snorlax is weakened it has much more raw destructive potential because STAB Electric + Hidden Power is much better coverage than Vap's Water STAB, plus it outspeeds everything.
It outspeeds Dugtrio and gets Baton Pass for getting out of there. I'm not entirely sure what the allure is, Zapdos seems much better as a BPer and as an all-around Pokemon, plus thanks to its Flying typing it has nothing to fear from Duggy, even when Para'd. I guess the lack of an Ice weakness goes a long way, and outspeeding Gengar, +1 DDtar, and tying Aero are nice things to do, too. Jolteon is probably at its best and most consistent in ADV from what I've seen, though.
Not even going to pretend I know how good it is in this gen, apparently it's good with Specs or Charge Beam + Baton Pass, and probably there was an era in an earlier more Speed-obsessed DPP where Scarf Jolteon was the bee's knees. I don't see it in too many DPP battles that I spec, so it doesn't seem to be top-of-the-line OU here.
Overall, while Jolteon made the jump to OU after RBY and stayed there, it never really transcended that level to be one of the major metagame-definers. I guess it came close in RSE, but really, it seems to always have been just kind of an off-pitch threat to toss out in OU.