I managed to hit level 75 on my Jedi Guardian yesterday while working through the Iokath story line. Yes. That's right. I haven't even started on the actual Onslaught content yet. Frankly, I'm not a big fan of the Iokath stuff either, but I just felt like I should at least start through it.
One overarching piece of feedback for the SWTOR team after working through KOTET and the first part of Iokath is please, no walker quests. Ever again. Unless you're willing to fix the mechanics in them, they are terrible, and please stop.
I'm debating respecc'ing the Jedi to a tank spec now, but haven't fully decided yet. I also started working my smuggler through the Hutt Cartel story line just to keep earning conquest points and gearing up. My plan is to do that and Revan and then skip straight to the Onslaught stuff if I'm in the right spot level-wise.
On my Sith Marauder, I've now finished chapter 2 and am definitely enjoying the class a lot more. I'm a huge fan of Jaesa as a companion, so we're just rolling through everything now. I'm also starting to unlock all the strongholds for the huge conquest bonus, so that's helping as well.
I haven't spent much time on the Jedi Shadow or the bounty hunter, but that will definitely change once I get the Sith Warrior done, which could happen fairly quickly now.
One other thing that I've discovered on the professions is that you can absolutely level the crafting crew skills strictly by leveling the various components used to craft gear and not by crafting any actual gear. Going to try that with the various ones that I've been working on. The big crafting project is likely going to require moving everyone's materials inventory into Legacy Cargo and then crafting away.