Based on my vast experience of running Esseles once and Hammer once... *lugs over a really really really big salt shaker*
Esseles seems like a game of jungleball. Run in, shoot stuff, nevermind tanking or healing.
Hammer seems a bit more real: you do want a tank and a healer. Things that greatly helped me:
* Turn on enemy NPC nameplates.
"Strong" and "Elite" NPCs have little additional icons next to them. Now you know who to kill last. (I believe in killing mooks first, since they are high damage / low health.)
* Non-Strong/Elite mobs don't need to be tanked.
This is a bit like being out in the wild: four Normal mobs standing around? Whatever. Nevermind marking: just rush in and KILL EVERYTHING.
* Quite a few trash pulls only have 1 Strong mob and a few Normal mobs.
Tank goes up to the Strong guy, bodypulling everything. Everyone else just gets 'em. This makes for a fast-paced run, which is lovely. Also, low stress on the tank: only one Strong icon? Run in.
* Blowing up canisters or pipes FTW.
Note that the first time you click on the pipe, it spills oil. Click on it again to actually ignite it. Of course, you could just blow stuff up instead, but where's the fun in that?
* CC is probably darn important.
I wouldn't know; we did Hammer with two on-level people and two people who were +6 or something crazy. But some of those pulls are big.
* Jedi Knights make Shadow tanks cry.
I mean, this is no different from dps Warriors making palatanks cry. I'M LOOKING AT YOU, GWYD. That said, once I figured out, oh hey, I don't need to tank the Normals? Then it was easy. Gwy goes jumping in, I taunt the Strong guy off him, profit. (To be fair, Gwy stopped doing that fairly quickly.
* "How do I get to the flashpoint?" Go to the Fleet, take the eastern elevator down to Dropship Landing Pad. At one end are lower-level monitors; at the opposite are instances more like 27+. The instance portal is directly behind the corresponding monitor.
Note that for people who have a security key, you can buy a Fleet Pass from the Security Vendor (Fleet, inner ring, Northwest) and then you should have an Emergency Fleet Pass in your General tab of your Abilities page. It has an 18 hour cooldown.