If you dont care about graphics. But you are all about mechanics, complexity and depth.
Anarchy Online. It is a VERY OLD MMO. Its first 200 (
) levels are free to play. The learning curve is pretty steep. But once you got into it, it is very addictive. Altought in the f2p version you wont have access to about half the game content. Alien raids (Aliens raid you and your guild city) that you can trigger, boarding raids, event based dungeon quests, 2 additional talent trees, huge amount of weapons and quests, personal and faction based research where you convert XP into from leveling into skill and ability researches, but still you will have acces to dynamicly generated personal solo or team missions (dungeons), a mind fuck deep skill and craft system, countless quests, planetary control pvp, level 20-60, 90-110 and 160 to 190 group/raid (depending on your level) raids, and then level 200 faction based world raids.