Fortunately, leveling another class for abilities is easier now than it used to be. Most abilities past the early 20s can't be equipped on other classes in the first place, and the very heavy utility skills are generally grouped early. Depending on your playstyle, you may actually find yourself struggling to come up with all of the cross abilities that you can equip, since your core class will generally have a large enough array of utility. But let's look at what abilities are generally useful for almost every class -- or at least useful to consider.
Decoy: Sort of the poor cousin to Featherfoot, Decoy is useful enough if you don't expect to be making heavy use of your MP but far less attractive otherwise. Considering that the class with the highest preference for Evasion makes big use of MP... yeah, balance concerns. It's not worth leveling for on its own, but it's nice to pick up on the way to better things.
Hawk's Eye: This, on the other hand, is well worth leveling for. Worth noting is that the enhancement to your accuracy lasts for a little while rather than just being an "on next attack" effect, meaning that it's to your advantage to bust out two or three skills in a sequence when this is active. It's especially useful on Marauders and Lancers, since they hit slowly and for big numbers, making a miss particularly painful.
Cure: Duh. Actually, Cure isn't my favorite way of having some HP recovery, but pretty much every class can abuse it in tight situations.
Protect: Also duh. More survivability is never a bad thing. Be wary, however, as there are certain enemies that love to steal Protect and essentially make your life twice as difficult.
Raise: Once again, duh. Solo, it's not all that useful, but you never know when it can find a use on the field.
Rampart: Gladiator has a dearth of useful skills for others, but Rampart is a solid addition to tanking rotations as well as solo players. In both cases, it provides a nice defensive boost, and tanks especially can benefit from the extra enmity it generates. It doesn't have a terrible cooldown, either.
Feint: Very nice for DPS classes, Feint gives you a chance to make up for a missed attack with a relatively powerful weaponskill. It's useful for everyone, really, but tanks will generally have better things to spend TP on, and it's not as damaging to them if they miss an attack. Lancers, Marauders, and Archers can make excellent use of this.
Invigorate: One of the big problems that Disciples of War have at the start of a fight is that their main resource of TP fills up only as they fight, leaving them unloaded for a while. Invigorate helps counteract that in large part. Using it right before a fight is often best, as that'll give you a couple of ticks of generation and let you go in with guns figuratively blazing.
Bloodbath: There was a time when attack classes could basically turn Bloodbath into an overpowered Cure with alarming frequency. Those days are gone, but Bloodbath is still a very solid ability that can help you claw back health in a long fight. It's best used in conjunction with Hawk's Eye to hit on the second or third attack in a combo string to gain the most healing.
Overpower: Tucked away high in the Marauder leveling path, Overpower gives you a nice conal attack in response to a parry. While no tank parries like a Marauder or Warrior, the parries still come, and this is a nice little area threat grab -- or just a good way for a solo character to do a little crowd control. Still, it's not as desirable a triggered skill for solo players as miss triggers are.
Featherfoot: There are partial parries and partial blocks, but there are no partial evasions. Featherfoot is especially great for Disciples of Magic, who have two problems while solo: a need for more MP and a dearth of any survivability. This skill addresses both, at least in part. Tanks should also be happy to grab it, as automatically dodging one attack every so often is always great. And if you can use it to duck away from a particularly nasty weaponskill, so much the better.
Second Wind: Without a doubt, this is the best skill in the game for keeping your health up. It's quick to execute, it's powerful enough to have a noteworthy effect, and it's on a short enough cooldown that you can use it regularly without worrying that it won't be there when you need it. It's even better on a Pugilist, of course, but every class can get a lot of use out of this skill. It's well worth leveling up Pugilist just for Second Wind.
Blindside: If you're doing damage, this is worth considering. Solo, it's a wasted slot. But in groups or pairs, it can have a definite impact.
Dark Seal: Thaumaturge, like Gladiator, isn't great for helping other classes. Dark Seal can have some noteworthy utility in dealing with enemies as a Conjurer, however.
What's not here
By and large, weaponskills that aren't tied to a specific trigger (such as Feint) aren't worth equipping. You can't get the combo effects of those skills, and thus they become weaker than your own abilities for the same TP cost. The changes have also taken away some of the most universally useful weaponskills (Moonrise and Circle Slash spring to mind).
There are also certain skills that can be equipped on other classes but provide no benefit for doing so. A perfect example is Provoke. The traits that make Provoke useful as opposed to Flash or Taunt are not available unless you're playing a Marauder, and the two classes that might want it have better options available. Sure, you could toss Provoke on your bar to supplement your other aggro builders, but why not just stick with enmity-increasing weaponskills? If you're hurting that badly for aggro, there are generally better options.
Sadly, attack spells have largely become useless outside of the linked class, compared to the previously limited utility. Scourge/Bio wasn't great, but it was at least a reason for melee characters to splash some magic. Oh well.
Think I missed a great skill? Leave it in the comments or mail it along to email@example.com with your own opinions. Next week, per reader request, I'm going to post a soon-to-be obsolete guide to making your start in Eorzea as painless as possible so you can start rolling in cash and fame right from the start.
From Eorzea to Vana'diel, there is a constant: the moogles. And for analysis and opinions about the online portions of the Final Fantasy series, there is also a constant: The Mog Log. Longtime series fan Eliot Lefebvre serves up a new installment of the log every Saturday, covering almost anything related to Square-Enix's vibrant online worlds.