Well, jarls definitely need to be weaker, whether indirectly, or directly.
The main problem is, of course, Odin jarls. They can get a +20% civ bonus, 10/15/20% from unit line upgrades, and 20/25% from myth upgrades.
Thor jarls can get extra armory techs, and a 20% myth upgrade; Loki jarls can get a 25% myth upgrade.
Statwise, jarls are fairly similar to hippikons, but superior in most areas - they have better armor (20/34 vs 10/25), better hit points (180 base vs 150 base), but less attack (9 vs 10), and they move a bit slower. They also get a big bonus against MUs. Of course, the downside is that they cost more - 10 gold - and are produced from a more expensive unit building. The additional gold cost must also be seen in the perspective of Norse getting dwarves; when you take into consideration the cost and build speed of dwarves, Norse will have about 5% more gold more than Greeks - and that increases later on, as pop becomes a limiting factor in economic growth - meaning that in real terms, jarls don't actually necessarily cost more to Norse than hippikons do to for Greeks.
The problems jarls presents are these:
-Jarls are the Norse counter for MUs that hersirs cannot get at; especially ranged ones. The problem here is that this actually makes melee MUs all but obsolete vs Norse, if jarls are to be able to counter the ranged ones, or vice versa, ranged MUs would be ungodly strong vs Norse if jarls weren't able to counter them.
-Jarls are nearly uncounterable. As much as ulfsarks and spearmen have problems with hippikons and raiding cavs at their base levels, jarls are even harder to kill; the spearmen lines generally only are cost effective at equal pop against them, not at equal cost - in other words, you'll spend more resources being cost effective against jarls, than the jarl (ab)user will spend on jarls. The dedicated counterunits don't really do that well, either - prodromos do well enough, but camels are so-so.
-If jarls become easier to counter, Norse might be sitting ducks for the aforementioned ranged MUs.
So, there's a bit of a hole that ES has dug there. Primarily, it's based on what I believe someone from ES once stated here - that they balanced the game based on the assumption that food is a "cheaper" resource than "gold." It'd all match up much better if that were true, but it is not the case at the moment, given that gold generally is substantially easier to get than food (barring extremeley hunting-rich maps and extremely fish-rich maps [which generally favor Norse even more, for other reasons]). Now, the specific problem at hand is Odin jarls. Odin jarls have more hp, and access to additional myth upgrades. Since the basic game is balanced with jarls being barely counterable at equal pop for the spearman lines, Odin jarls getting extra hp breaks this pattern a bit. Of course, myth upgrades like Bragi and Horus get outweigh the combined power of all the myth upgrades that Odin can possibly get for his jarls.
Horus is a mythic age god. If you're playing Egypt, and can survive until then, you'll be able to win. That's a pretty big "if," though, as Norse are better at map control in general than Egypt is (except Set, but Set's too strong in general anyway) and Isis doesn't get Horus anyway. Ra either has the weakest econ or the weakest scouting in the game - in any case, Ra doesn't have the protective features of the other Egyptian civs (shielding monuments or animals), and he's basically sitting rush bait for Norse. Camels aren't really cost effective vs Odin jarls at all; their greater speed isn't an advantage at all. Camels also die badly to TA.
Bragi is available to Loki and Thor. Odin owns Thor in classical anyway; Odin will beat down Thor pound for pound. Loki is a different matter altogether, and he will own either of the other Norse in classical in most matchups. Odin doesn't get Bragi. Of course, Odin gets jarls - so, logically, since Odin can't counter the jarls he gets, Odin vs Odin matches should logically eventually degenerate to jarlfests.
None of the Greek civs get substantial enough myth upgrades to counter the full weight of Odin jarls for their hoplites. Hoplites are stronger per pop than the units the other civs get, of course, but still not strong enough, or possible to mass fast enough. Hoplites die horribly to TA anyway (TA have an extra large modifier vs hoplites for some reason - meaning hoplites are essentially "better infantry that isn't really better"), so no sane Greek player would make them. That leaves prodromos. Prodromos might work for Zeus and Poseidon if they go Hermes/Dionysus. For Poseidon, that should put his prodromoi at levels similar to base levels, though not quite base levels. The problem, of course, is that prodromos cost food, and are thus expensive units in general, and take 3 pop, so they basically suck against anything except cavalry. Hades doesn't get any myth upgrades for prodromos; his prodromos won't do well enough to make a difference. Now, Zeus also gets a second option beyond prodromos in mythic, in the form of myrmidons. However, those come into play late enough for the jarl player to already be winning big.
Beyond all these options, all civs can make towers with crenellations, which should drive even jarls away. The problem with that approach, of course, is that it only delays the inevitable, and is a good way to cede map control to the Norse.
So, I'll bet some of you would come up with the brilliant response of "but you're saying that jarls are counterable, then." No, I'm saying that most civs can counter Odin jarls, but that they're facing an uphill struggle to do so. Also note that I am not at all talking about "fast heroics" or such strategies; I'm talking about games with regular progressions to jarls; and, frankly, Odin has other advantages that are more imbalanced overall, when you consider a regular progression.
To summarize:
Ra - Horus spearmen, but will be dead before that.
Isis - nothing.
Set - Horus spearmen, but will either kill Odin early on with the help of animals, or die trying.
Zeus - prodromos and myrmidons. Likely to die in classical, though, due to raiding cav being overpowered.
Poseidon - prodromos.
Hades - nothing.
Loki - Bragi ulfsarks. Will generally kill Odin in classical, though.
Thor - Bragi ulfsarks. Will generally die to Odin in classical, though.
Odin - can make jarls of his own. "Fun."
Ok, so the next item beyond jarls is Odin's overpoweredness in other areas.
The combination of the best early econ bonus on most land maps, the ravens being able to scout and keep track of enemy vills, as well as enemy army movements, raiding cav being too good, and the regeneration bonus makes Odin a monster civ in classical; Set and Loki can beat him, most other civs are generally left behind.
That is part of the overall picture, of course - the bottom line remains that at a certain phase in the game, Odin jarls are impossible to counter cost effectively for most civs, and hence too strong.
Jarls in general need to be toned down a bit.
I would suggest to give hoplites, ulfsarks and spearmen a multiplier against all cavalry (1.2 to 1.4 or so), lessen TA and axemen multipliers vs infantry (and TA should not get a special extra bonus vs hoplites). Further, raiding cav hack armor should go down by -5% to 10%, and jarls should similarly get a -5% to their hack armor. The Odin hill fort bonus should be decreased to +10% hit points. With those changes, Odin should be balanced, and his jarls will still be strong, but far from overpowering.