Is there quest grind or just grind? Grind and some quests to do.
Large playerbase? Yes, there is quite a few players.
Instanced dungeons? Not really that many dungeons, but i dont believe they are instanced. There is clock work though.
Nice interface? I quite liked it when i played.
If you want to know my experience i got to level 80 or somewhere around there so i played quite a bit.
I would recommend this game if you have someone to play with. It is highly dependent on a party to level efficiently. I used to play before but I stopped as I had to spend an hour or so looking for a party. Before I would just quickly find a party with people I know.
Pretty much what Aku said, you can solo for maybe 1-20 but once you hit around 30+ the grind gets stupidly hard and partying is the only good method at that point.