Have a piece of gear that could be an upgrade if it had that one stat? Enchant it! chances are you'll get something added that will make you say "Hmmm, now that helps a LOT!" if you don't like the enchant you got, Disenchant and try again... there's your gold sink, trying to upgrade your gear, not insane repair bills (although in D3 I knew how to kite so that was not too much of an issue)
Even with gold sinks like that gold is going to eventually become worthless, just like it was in Diablo 2. Diablo 3 may not have enough in-game gold sinks (not to mention the problem with crafters generally being worthless, so never used), but gold is at least worth something and is valuable.
I don't see TL2 having longevity for 2 reasons, history and game style.
First of all, if we look back at the history of TL1, it only had a few patches then was abandoned by the devs to develop TL2. Now you may say, "Well, yeah, TL1 was just a 'trial' of sorts to get a working model, TL2 is the REAL game." I understand that, but the devs will be working on the TL MMO now. TL2's proceeds are going to fund that. We're in the same situation as we were with TL1. I expect there to be some patches to the main game, but I expect most 'patches' will come from the player modding community. I'm glad TL2 is cheap, because players will be working to fix it themselves.
Additionally the play style of the game doesn't support longevity. Awesome drops everywhere are fine and good, but ultimately make the drops worthless, because everyone gets them. I'm sure for a little while they are great and help with progression, but they'll be useless to trade (keep in mind everyone getting similar drops, makes it so your probably won't be better than another person's drops) and eventually even the most powerful Awesome Annihilator Swords of Devastation will be quite common. Character building, while I really like the idea of set stat/skill points, it only promotes hard build diversity, not soft diversity. You're going to have around 4 specific builds per class that work at all. This is most obvious in a game where you -could- use a 2-handed sword, but have of your skills you chose are for ranged weapons, or you could dual wield, but now your shield skills are useless. The only way to try out a new skill/weapon effectively is to re-roll. Now I know people will say that Monks can't use bows, so you can't make a monk bow build. I know people will say there's really only one monk build that works anyway. There is a big difference here. With a monk I can make myself more tanky for my team by simply switching skills and throwing on a shield. If I play alone, dual wielding works better. When I am running a friend through the game, I use a 2-hander and tempest rush. My monk is "My monk", he's not "My Tank-Monk," or "My Fists Monk" or "My Stick Monk". He's just mine, and I only needed to build him once and he does everything I need. For me, that ultimately saved me time and has made the game more enjoyable. Enjoyment, for me, doesn't come from making a build that after 50 hours of playing doesn't work for !@#$. It comes from making something that lasts and I can remodel myself to make it work. TL2 doesn't hold much interest for me, because I just don't see it lasting.
Wow, I'm rambling.
TL2 doesn't hold much interest for me, because I just don't see it lasting.