Constitution is what I vote to be most important overall, as every single character needs it.
I usually play my favourite class, wizards, so an Intelligence of lower than 18 is usually horrendous in my eyes.
Second favourite class is rogue, so Dexterity for that, and Charisma. Makes for some good fun.
Also, DDO isn't the only game to use a Dungeons & Dragons rule-set. There are games which have come earlier and done it better. (Neverwinter Nights as a single example.)
Not an insult, as I like DDO, but it doesn't need to be mentioned at all.