Impressions:
Diablo 3 is great. It still has a very Diablo feel while making it's own mark on the series, most notably with the exceptional graphics. The gameplay is fast-paced and the loot is plentiful. The skill system seems simpler from Diablo 2, but there is still some customizing once enough levels are raised. All of the classes are unique enough that there will be a lot of replay value, I loved all of them. This will be a definite preorder for me.
TERA is not as great, unfortunately. It really tries to set itself apart in a genre full of clones and it just isn't different enough from the vending machine MMOs that keep coming out. The combat is a little faster paced than many of them, but the charm of it wears off after the first few fights honestly. Many will get sucked into preordering it and playing for a month or two but probably not long after that. It's a "flavor of the month" game and not much more.