just going to offer some quick thoughts on a few months of playing Guardian, Warden and Burglar (in order of most to least).
Guardian is very straightforward, pretty powerful solo and in groups, easy to pick up if you want to tank. Tanking is a little tougher to manage than in WoW in the sense that threat management can be a little more mysterious, since there's no interface addons to show threat levels for everybody in the party. On the other hand, nobody is so squishy they die in a single hit. The class has a fun core mechanic built around blocking/parrying and then opening up a series of attacks based off one initial counter-attack. This class's tanking style is very upfront and snap-aggro based, in the sense that you will build threat in chunks with attacks as you land them. Guardian is a little bit of a stereotypical knight kind of class, with the heavy armor, and either swinging a big two-handed weapon (especially when soloing) or using a 1h+shield combo to smack enemies around.
Warden is pretty complicated and I highly recommend soloing it as far as you can go. I played it in a near-permanent group with two of my friends and it made it pretty tough to play. The reason is wardens build combos, and for most mobs in a group you're never going to build up a big combo to use your finishers. That said, wardens have way less buttons to press than guardians (3 different combo skills that you build into finishers) so it becomes more about memorizing how to build combos properly. It's not really like a fighting game. Also it's stronger solo because they can stack up a ton of self-healing abilities and damage over time abilities on the mobs they fight, which lets them take on really tough monsters pretty easily. That said, the one drawback I noticed with this class is it uses a LOT of power (MP, the blue bar) so you want to find some ways to address that through traits and higher level abilities. They don't deal as much snap-aggro as a guardian, at least when I played (up until April of this year) but for a long fight they are way tougher to steal aggro from. They also get tons, and I mean tons, of cool personal utility things like a travel form that more or less negates the crappy first mount you can buy, and lots of personal teleports (LotRO is a low-magic world, so you aren't literally teleporting, but it's how they function) to virtually all major cities in the world.
Burglar is pretty cool and the kind of class I wish the WoW rogue was, at least to a certain extent. You have stealth and dual wielding, you have pickpocketing and other rogue-related tricks, and you have a ton of cool debuffs and other things you can trigger so that you're actually pretty resilient for a light->medium armor class. Unlike the WoW rogue, you aren't built around spamming one or two abilities to get to 5 combo points and then unleashing a finisher. You just choose the right attack for the situation.
I'd also say the easiest class to play for a beginner is either Runekeeper (very straightforward and pretty good damage early on) or Hunter (excellent damage and simple to figure out) even though I didn't really like either class much.
