If you are starting from scratch, there is ALOT to do now. Enough to keep you busy for a few months, and you will have company, as there have been more and more people coming back.
The big thing though is, if you had issues with the game play, or the lack of content, you may be pleasantly suprised. However, if you had an issue with the UI, Market Wards, general item availability, lack of party mechanics (especially at low levels), linkshell management, crafting mechanics (different from actual crafts), gathering, or lag, then you will find the game is much the same as it was at release.
They have redesigned the characters, battle mechanics, mobs, mob placement, leve system, abilities, and added several quests and dungeons and all levels from ~1 to 50. With there being more to do the higher you get. You definitely can't level off of questing alone, but, after getting a few levels, you can always take a break from the grind and complete some quests.
Most of the open world quests are simple kill quests (not all). Very few have cutscenes (the current favorites of the new ones are definitely the inn quests, then maybe the Black Mage and Warrior quests).
There is something to do in every town, and while people still gather in Ul'dah, there is no longer a need. You can either put up your flag for a party, or search for a party and join (they added the basic functionality from both features in 11, however, the LFP feature is definitely not complete, but the Find a Party feature actually works, unlike it did in 11).
The Market Ward is now a complete nightmare, since the server merge, everyone is still putting their retainers in Ul'dah, and to be honest, I haven't sold a single item since the merge (via retainer). But, it is definitely easy to find stuff and buy it:P.
Anyways, if you have money to burn, and time to burn it in, there are worse investments:). Add to that there are things you can do now that you will not be able to do after the release (get a Goobue... which is or is not a good thing:), or take part in the Live events, they are just quests that appear with each update that lead into the end of the 6th umbral age, which give items, and exp, and right now are just defense fights, and require level 45+).