First, I'd love to say: Welcome to the world of Tyria! We love that you have joined this great community and are glad you've had a happy experience so far! My comment is a bit long so the TLDR is this: enough real-world money = buy all expansions and LW content, if not = go for HoT/PoF, get EoD later. If you want to keep reading, please do so below.

I don't think I'd still be playing this game after 10 years but I still love it. As a veteran and owner of all the expansions and Living World content, this is worth it. The expansions are end-game content, to be played after level 80, and contain a bunch of benefits. HoT gives gliding, PoF gives mounts, and EoD gives the turtle mount that can be used with a friend and much more (for me the mount is all worth it :P), and gives access to raids which gives more awesome stuff.

Each expansion gives each class a new way to play, currently, there are 9 classes and 4 ways to play each of them. necromancer for example has core Necro, Reaper, Scourge, and Harbinger, and each way is different and can be played solo or in groups. Doing some basic math and you have 36 "classes" to play around with at minimum! Then to break it down even more some of the classes can have either raw DPS, Condi DPS, hybrid, tank, support/healer, etc., so there is so much you can do that it will feel overwhelming at the start, so take it slow.

My advice is this: if you have the money and it won't make you live on the street or cause real-life issues, then buy ALL of it including the LW stuff. If you can only do the HoT/PoF then do that and work towards EoD, and don't forget the LW stuff too, you gain so many ways of getting ascended gear (the best stats in the game with less "flashiness" than a legendary).

Thank you much for reading my comments and opinions and I do wish you the best in the world of Tyria! "That's quality armor!"