To me, the real problems with Jolteon are:
a) So 1 dimensional (come in, spam powerful electric attack)-- there's really not much to its offense besides this. Everything else can carry is just trying to be almost useful.
b) no real resistances (this is the REAL crux of Jolteon's problems)
-the fact that the only resistances it has are Electric and Steel (edit: And Flying, though not too many Flying-type attacks in OU Jolt could switch in on...) is a big problem (especially when the vast majority of Steel moves are physical and beat on poor Jolteon pretty damn hard)
With everything hitting so hard, Jolteon's got basically zero opportunities to come in--
This is a distinct difference from Pokemon like Gengar, Thundurus-T, and Starmie (or even Breloom), who despite having weak defenses still get opportunities to switch in. Even Espeon and Alakazam, thanks to their spiffy new abilities, have chances to switch in to things like status when few other Pokemon would want to.
Jolteon's only real opportunity to switch in is on Electric attacks-- but except for Rotom-W and Starmie, Jolteon doesn't have a strong pronounced advantage of users of electric attacks / Thunder Wave-- not compared to most Ground-types anyway. Moreover, against most electric users (bar Rotom-W and Starmie), Jolteon can't do what it does best against them-- spam its Electric attacks.
Most teams will have an electric-absorber in a Ground type, who generally pack a lot more bulk/resistances (And a lot more opportunities to switch in with advantage against more foes)
The issue of not having many resistances is a big one in BW; because chances to come in on a kill are fewer and fewer-- and everything hits harder. Furthermore, with even more threats to contend with, the few spots on a team with which to tie together a cohesive net of checks against an expansive threat list are even more valuable. A Pokemon who can't bring any defensive value puts a real strain on everyone else.
Therefore, Pokemon, like Jolteon, who have little defensive merit-- Pokemon like Infernape, Weavile, et. all have a real hard time making it on teams.
Looking at the tier list, there are NO Pokemon in the S or A ranks that lack useful resistances except for Alakazam, whose ability gives it other switch in opportunities and extends its longjevity.
The highest ranked Pokemon on the list with little defensive merit are Dugtrio and Gothitelle, who are trappers with a very special Niche. The next highest ranked is Weavile (oh, another trapper).
Once you get down to C Rank, you see a LOT more. But the point stands-- not having some combination of good bulk or useful and usable resistances is a BIG set back.
Basically, if you don't have useful switch-in opportunities that give some defensive benefit to a team-- you will have a HARD time finding a team slot in BW/BW2