Jolteon was just one of Eevee's evolutions. Compared with other Pokémon, Eevee could count as one of the most remarkable: although it was among the most common Pokémon, it had more ways to evolve than any other.
Compared with Jolteon, Eevee in its original form was much weaker, serving only as a starting form.
Honestly, if the developers' darling Eevee hadn't received an exclusive Z-Crystal that made it a uniquely explosive setup user capable of passing its boosts, I probably would have skipped it altogether. As the base form of eight fully evolved Pokémon, and a popular Pokémon at that, Eevee ought to have the greatest variety of... breeding stock.
As an unevolved Pokémon with evolutions available, Eevee naturally didn't have particularly high base stats, and its distribution was very evenly spread, unlike Gastly, whose stats screamed fast attacker. On the physical side, it had Adaptability Return, the strongest Return among all its forms... not that it was worth much. Still, people once used it to poke fun at the old "one true king," Flareon, whose filler Return was weaker than its own pre-evolution's.
With maximum investment in its base 55 Speed, Eevee after an Eevium Z boost was just barely outsped by Choice Scarf users with base 90 Speed, but it outsped Pheromosa's base 151 Speed. Pheromosa posed a major threat to Eevee, so investing enough Speed to outspeed it after boosting was essential. As for Abilities, Run Away did nothing in battle. Adaptability increased the power of Eevee's Normal-type STAB moves, but was useless if it didn't carry any. Anticipation could detect whether the opponent carried a Fighting-type move or a one-gigabyte Explosion, occasionally helping with decisions. If the opponent had that one-gigabyte bomb, for instance, you could put up a Substitute after boosting.
With Eevee's Z-Move base