I know that in GW1 with the buy to play format they use you had to buy each xpac separately so I assumed it was this way here.
Yeah, the two actual "expansion" packs are purchase only, but they are bundled together right now. I think I got them for $30 or so a few months back. But the episode releases are free as long as you log in around the time of their release. I think it's fair, and you don't HAVE to buy them. You just lose any sense of story progression, assuming you care about such a thing. Once I had hit 80 I immediately went to Path of Fire to get the Raptor mount and decided to finish the expansion. I still enjoyed it even though I didn't get the past character and event references. Whatever, I'll go back and do the other stuff later.
So, a few months ago I had started playing this game again. I played it briefly back at release in 2012 and just couldn't get into it. My main complaint being that the combat felt rather repetitive. I'm not sure if this is true for all "Core Tyria" professions, but things definitely changed with the elite specializations. All the elites I've played with have felt really engaging and fun. Combat feels like it's pretty damn deep once you start digging into the details.
I highly recommend this video series:
This is a little dated since they've had a balance patch since the video releases, but I think its probably still 95% accurate.
For me, all I'm interested in right now is world PvE. And I am having an absolute blast with the Flamethrower Scrapper (engineer). I may not do the most amazing damage, but my god I can take a beating. I'm not sure if there is any profession + build that's any more face-tanky than the Flamethrower Scrapper? I can literally take on 3 veteran fire hydra at once in PoF without breaking a sweat. Maybe that's not a big deal for some other professions but the others I've tried I couldn't handle that scenario. And that's part of the fun for me is challenging myself with the various tougher enemies out there.
Anyway, I've been really, really enjoying the game now. I absolutely adore it, actually. It has so much character. And I've come to really appreciate the "horizontal" gear progression. It's nice to just grab some exotic gear and have it be "good enough" for most content. And crafting ascended stuff isn't that big of a deal thanks to gw2efficiency.com
Also, holy shit, are the world dragon encounters in this game epic. The first time I did Tequatl the Sunless I was amazed.