I couldn't really explain truly what I love so much about HZ. But I'll try. Perhaps some background as to my MMO experiences will help.
I started out in MMOs 5 years ago, with a free server of UO. I still miss UO, I would love to play it again, but I don't wish to start over. I've tried, many times. I played the game for 3 years, maxing a lot of skills (including the ever-evil Taming) though on our server it was easier than OSI. We had a very small, very friendly community and I loved the freedom. I admit the first time I tried it, I was put off by the "here you are, in Sosaria... do what you want" with no guidance, but once I got used to it and got a feel for what you could and couldn't do, I loved the fact I could go anywhere and do anything (at my own risk!). The crafting was fantastic and I loved that to raise a skill you had to actually practise it, rather than beat a monster with a piece of wood and magically become better at spellcasting, melee and healing all in one go You could own a house, and design it yourself, the place vendors around to sell your goods! Spellcasters were powerful but weak, tamers were strong but hard work, you could be a treasure hunter or sit fishing! Or become an evil thief, murderer or beggar. The game had freedom. It had reward as well, every .1 gain seemed like a huge achievement, particularly towards GM/Legendary status.
Eventually our community started to break down and our host couldn't keep the game running anyway. I moved on. I tried OSI but couldn't bring myself to level everything up again without my friends, and there is so little housing space left it just didn't seem worth it. I moved on. Many free MMOs were tried but eventually I gave in and decided to pay.
EverQuest 2. My first 'linear' MMO, and it was still fairly free. Quests seemed a novelty to me, and it was a shame not to do every one that I came across, which meant I was constantly trying to finish quests and got pulled along in the endless grind for more exp. Crafting was pretty fun, though I found it far less enthralling than UO (and I believe has been dumbed down now). The classes were intriguing, but being stuck to doing 'one' thing annoyed me, and I dislike the whole "yay, I levelled... suddenly I can cast a fireball despite never having tried magic in my life" model. I had fun with my then-boyfriend but we got horribly bored at level 26 never seeing anything for our efforts (and in fact losing progress if we died). No real 'pet' class like the tamer from UO disappointed me as well, though as a Warden I had fun running round as a wolf. So after just a few months, we moved on. Housing was a disappointment. One door, many instanced houses. Sure you could 'decorate' but you didn't own a piece of the world.
Again, more trying free MMOs, too many to remember. We got briefly addicted to Silkroad but it was too grindy. Not enough customisation. WoW was due out, as was Guild Wars... I picked GW due to the no-sub model, got into the last beta weekend and have played it from the beginning. I enjoyed it. I still do. I treat it as a singleplayer game though; instances don't give a huge world feeling. 3 years today in fact I've been playing. It's really not an MMO though.
I found HZ while playing GW and was immediately enraptured. Dragons, and not only dragons but flying ones! A huge world, a persistant world, with events and adventure and crafting and housing. Crafting to rival UO! The house is not just planned and then bought with gold, you actually create each unit and apply it, watching the scaffolds until they pop. With higher levels you can create different looking swords and put different stats on them (of course still limited but it's there, my longsword can lookd ifferent to yours). Friendly people who aren't just interested in where the next artifact/epic/gold piece is coming from, but in actually making new friends and acquaintances, and helping out those new to the world. The detail on some of the animations is amazing, despite being outdated and if someone casts a big, impressive spell, the whole world seems affected with the skies going grey or leaves being whipped up...
... and then Unity died. We were abandoned for over a year and a half. I didn't know what to do with myself. 6 months of HZ and I was already gonna miss it. I tried yet more free MMOs and failed on them all, hating their simplicity or lineality (is that a word? heh). I attempted Vanguard and in all honesty think it could potentially be the best MMO out there... unfortunately SOE got their hands on it. It's a beautiful looking game, I love the crafting and diplomacy, the fact it has truly 'built' houses like HZ... but it runs badly and if you attempt to go to an area you shouldn't, you have next to no chance of even running through, which cuts down on exploration of this amazing world. A month was all I could take. I tried it again recently... I loved it til I tried exploring and came across the same problems. Though few things will ever beat me climbing the mountain next to Tursh and seeing the huge fort with dozens of dragons circling it.
I tried LotrO from the day it opened its doors with my boyfriend. We had a lot of fun, made it to about level 23ish (seems to be the standard area for me to get bored... hehe) but it was just too grindy, too linear still, crafting while good was not fantastic. A couple of months, if that. A great game and certainly no worse than WoW, in fact I think it better, it just... is still the same old "level up, gain x skill, be the same as Minstrel A".
I finally gave in to WoW, a free server with a friend at first, before moving to official servers to play with my deputy manager. And I've been there ever since, 1 year 4 months. I have a level 70 holy paladin, we just started raiding Kara. I met some interesting people in game, mainly idiots though. Luckily a lot of real life friends started up a guild and we got a bunch of other nice people in there, but when I log in there is nothing to do but grind for gold to get my epic flyer, or go raiding. I don't craft. I took enchanting simply so we'd always have one in 5 mans. I have mining as it's good money-maker. Crafting is not only simple, it's pointless (unless you're a clothwearer or an alchemist). Everything requires you to be logged in god knows how many hours a day just to achieve anything worthwhile. No housing. Instances... ok so it cuts down on ninja-looters (theoretically...) and spawncampers but... well it's not a truly persistant world then.
2 years after HZ first captured me, my dragon is reawoken. 1.5 years in the void. She'd have been played all that time had I been able to. I even restarted nearly a year ago now on Chaos, though I rarely logged on. She's been awake 10 days now. I haven't logged into WoW once and will only log in to raid (and then, only because my real life friends will whine at me if I don't). I'm not going to renew my sub when it runs out. I'm back in Istaria and I don't intend to go. I have another 48 levels of Lairshaping to go, and 23 more adventure levels. And that's just on my Lunus. I have a Helian and another Lunus to raise, plus 4 bipeds with multiple craft and adventure classes planned. I have a fantastic guild, a great server and a bright future. I can do what I like, go where I like... it is as close to true freedom as you can get.
I still think that UO is the best MMO out there, though that's probably due to the fact that I have met a ton of great friends that I still talk to, and that it was my first MMO. Horizons is very, very close to second on my list, if not joint top. They're the only two MMOs I've ever had the pleasure to play that truly give a sense of freedom, of being a character in a world, not simply a bunch of pixels interacting with more pixels. I've only ever roleplayed in those two games; I can't get a feel for characters in the more linear games. My first GW character has a backstory... but only really because she's Pevil, and Pevil has an ongoing story. Here I feel my characters; know just how they would react, what they'd say and can envision it. They actually live, and so does the world.