Are tanks complicated to play?
They're more complicated, from a purely tactical sense, than DPS or heals. As such, if you would have to label any of the roles rather than role/class combinations as "complicated" it would have to be tanks. This isn't to say that it's difficult. It's simply the most complicated of the relatively simple role based skillsets which boils down to generating threat quickly and across all or most targets in any given fight while maximizing your survivability.
Depending on the class in question, these 2 things (threat and survivability) can either be incredibly simple (threat for Shadows or VGs; survivability for Guards or VGs) or remarkably skill intensive (threat for Guards; survivability for Shadows).
Is it mostly to take a defensive skill tree and then attack, defend and taunt?
All of the tanks have a reasonably well defined playstyle attached to them in order to make them effective, just like every DPS class and spec does. If you go into a fight without understanding what the intended playstyle for a class is, you're going to have a bad time. You can push this a bit with a VG since, as long as you're properly facerolling Stockstrike, HiB, Energy Blast, Ion Pulse, and Hammer Shot, it's kinda hard to go wrong. The playstyles for Guardians and Shadows are a bit more convoluted and not worth explaining without knowing which class you actually intend to tank with.
Is there any advice anyone can give for playing a tank well?
Tanks require the absolute most situational awareness of all of the roles. You have to watch and pay almost constant attention to every enemy currently in the fight with you while still beating on things to make sure they're still attacking you. If you don't pay attention, you end up with free enemies that will render your healer into a corpse. As a tank, it also behooves you to understand fight mechanics better than any other class, especially on a Shadow. You have to know when and if to burn a survivability CD, where to drag enemies, where to turn enemies, when to Taunt and when not to Taunt, and all number of other interesting and weird situational things. If you already know the fights you're dealing with, it's not really all that much more work than DPSing. Personally, I find it easier since I get to take charge rather than having to follow someone else's lead, and I get bored when simply DPSing.
Also, make sure you know your class very well. A bad VG isn't going to get his group killed by not getting threat or dying too quickly, but he'll bottom out his ammo and sit there barely dealing damage and generating half of the threat he should be for the remainder of the fight. A bad Shadow isn't going to likely lose threat (unless massively overgeared; Slow Time takes care of most of that), but she *will* fall over and die way faster than she has any right to (debuffs, self heals, self buffs, and CDs are all major components of their survivability which are relied upon extensively; if you don't utilize them properly, you gonna die). A bad Guardian is going to have a hell of a problem keeping aggro because he's going to have all kinds of problems managing his resources; if you can't manage your resources, you can't really attack, which kinda renders your roles as a tank almost completely redundant since your effectiveness is predicated upon actually being attacked.
Once you've learned the proper way to play your respective class, there's a few things to keep in mind. First off, don't use Taunt on a target that hasn't been hit yet. Taunt's primary utility lies in the fact that it provides you with enough aggro pull off of whoever it might be attacking at the moment; if the target hasn't been attacked yet, you're not gonna generate any threat with Taunt which, while it *will* make it attack you for 6 seconds, won't do bupkis for the rest of the fight. Secondly, at the start of the fight, go out of your way to tap every single non-CCd enemy at least once so that they don't decide that the unprotected healer makes too appealing of a target. This is often very easily accomplished by dropping your major AoE on the largest cluster of enemies early on in the fight; just make sure you don't accidentally break CC when you're doing it. After that, your best bet is to work your way *down* through the enemy ranks (champ>elite>strong>standard), getting aggro on anything while making sure that your DPS starts chewing through their way *up* through the enemy ranks (standard>strong>elite>champ). DPS that decide to go outside of the proper kill order can make your life *very* annoying, which is why idiot DPS are the no. 1 cause of tank aggravation. Get those 3 things down and you won't get many complaints.
Something else to note is that, in many PuGs, tanks are the de facto assumed leaders of groups. Many people will expect you to tell them what to do and charge in first after checking with everyone else. It's not a definite responsibility, but it happens often enough that it's important to bring up.