The difficulty spike over the last GW had me going crazy at first. Once you get to the mainland, your not in Kansas anymore. 320 damage from a Dragon's Stomp anyone? Yeah... After some worthwhile playing, I got to love the new difficulty. Every new group feels like a challenge now. I actually feel fear when approaching a group! Excellent work

Even the material shortages were fun. It completely sold me on the dynamic economy of the game. The runes also. 16 player missions!! Loadsa' fun.
The ritualist class is just fantastic! I truly am in love with the play style. Part Necro, part Monk, part Ele, and then some. The item skills are a brilliant idea. As are the weapon skills. I have not had the opportunity to try out the assassin properly, so I'll reserve judgement. I will say that teleporting is far too cool. Especially the effect it has on item spells.
example:
cast Tsungrai ashes. Spirit Walk to a spirit you created earlier (at the rear of the battle or something). Bam! Out of aggro, automatic drop of the ashes, resulting in instant, high-end healing.
Or for Offence. Cast Daoshen ashes. Deaths Charge too the nearest group of foes (or Spirit Walk if they use spirits) Bam! As soon as you port, your priority is to get the hell out of there, as your ashes have dropped automatically. Ingenious design. Also, unlike Ele's with adjacent, Burst spells (crystal wave, aftershock, flame burst etc.), there's no stupid pausing either.
The art:
Fantastic. The Ritualist animation is lovely. Casting spells has never looked so good. Their overall visual design just rocks. The quality of the environments are a fine achievement. The beauty of the first island is easily on par with Pre-Ascalon. If not surpassing it. The stark contrast of the mainland is also refreshing but equally as beautiful. It's like China Town on steroids. Great stuff. As an artist myself, this is really important for me.
However, the monsters I have seen so far aren't that great (I'm only up to the mainland). I have always hated the whole Doom flesh-metal thing. It just reeks of little imagination (something Anet can't really be accused of in other parts of the game, but can sure be blamed for here) hopefully we'll get creatures that are as cool as the Bladed Aatxe and the freaky Storm Riders later.
Also the animation and the general look of the assassin's kinda' sucks. They just don't move right and their bodies are uninteresting (both male and female). Some of the later armour they get looks cool though.
Still, Factions on the whole is a great achievement.
What else did you guys like specifically?