Heh, I remember Horizons / Istaria being an off-shoot of Mordor / Demise games where the original creator left. I've never played Istaria though, but may be it's something worth looking at.
I still enjoy Istaria, but ended up leaving for a number of reasons. The first being that it is still a subscription game... Which I suppose is understandable because they have multiple servers and need some income to keep them up and running.
The second being their leveling and class system is in serious need of an overhaul. I have a love/hate relationship with Horizons' classes, mainly stemming from their almost mandatory multiclass feature. In order to succeed it has become common practice to level a base class like fighter/scout/cleric to 100 before trying any other class, merely because you get to keep some of the "masterable" abilities when you switch your main class to something else. This mindset is so ingrained that players and Devs alike don't seem to want to fix individual classes so they can stand on their own. Whenever I'd bring something up about it on the forums players and devs would just tell me to level cleric (or whatnot, but normally cleric for the heals) to 100 before I try any other class. WTF?
Thirdly was their community. While I have nothing against them, they are not low level player friendly. Most of them have been playing the game for upwards of 5+ years and have maxed out many classes. There are rarely any low level players to team with or RP with and most of the veteran high level players are to busy crafting to aid you with anything.
Lastly, it was a GRIND. Their crafting system is AMAZING and I still look to them when comparing new games. However, XP comes at a snails pace when doing anything (especially crafting) and often times you'll spend an entire session of two or more hours collecting resources and refining them.
In the end, I love it. What other game lets you play as a Dragon? If it went F2P I'd pick it back up in a heartbeat, hell. If I had more disposable income I'd pick it back up

-Q