With stunning graphics, great soundtrack, and solid/innovative combat system, Tera is amongst the best MMO's ever!
The music of Tera is also great, with beautiful tunes that match perfectly the combat (changing the tune according to the difficulty of your foe) and the different environments. Music is very important in games in order to impress the right mood on the player in each situation and in my opinion Tera is spot on!
However, the main feature that Tera possesses is its innovative (comparing to most MMO's) combat system, which gives an entire new dynamics to mmo combating, making it more fluid and engaging. Most MMO's are based on a combat system based on commands only. Tera mixes commands and movement together, akin to single player RPG's. You will have to learn the movement pattern of your enemies and dodge etc, as well as attack with the right abilities at the right time. This is a breath of fresh air if we compare to most mmo's in the market. Players can choose whether to use the classic keyboard+mouse or a gamepad (which adds to the console feel that Tera has). I play with an xbox gamepad and I really enjoy it. You can allocate most of your abilities to the buttons as well as a combination of them, which really enables the gamepad to contain dozens of abilities.
Another important aspect in Tera is politics, which allow guilds to run for elections in different areas called Vanarchs. However I have not experienced it yet, so I can't comment much further... It seems interesting though.
The only problems that I can perceive in Tera, having in mind that I have not reached end-game yet (so I can't comment on that) is that most quests are fetching/escorting npc's/gathering. The pattern is quite repetitive, however the combat system makes it very enjoyable. I haven't seen any serious PVP yet, apart from the duels :P, However I assume there is a battleground system which will be unveiled in the near future.
Also, there are already the odd trollers and spoiled brats. But most of them are harmless and one can always ignore players if needed. Overall the community is polite and funny.
Overall I recommend Tera if you are a MMO fan and especially if you enjoy the jrpg aesthetics. Many may think that the characters are too revealing/bold with their low waist trousers, opened overcoats and cleavages. However, I am not afraid of a bit of skin and I actually enjoy the whole aesthetics... Tera is really worth trying if you are seeking a new, enjoyable MMO.