The main incentive they need is creation of classes/jobs that fill the tank role which look like would be extremely good fun to play. I never played a tank class/job when the only options are PLD and WAR because those two bore me both in concept and in how often those classes are the same ones in most MMO's. Paladins and warriors are the generic go to classes for most MMO's when making a tanking class.

I will however be making a tank character when DRK comes out because it is not overused in MMO's and it sounds more unique and interesting. Same reason I did not play a tank PLD or WAR in WoW, druid was a more interesting and unique tank option as was DK which is why I made those instead. The lack of tanks issue will go away when DRK comes out for a long period of time but then will have a lack of healers still until a new healing class comes out.

I do however agree GC seals are not a good incentive for getting high level characters to run low level content. Most high level people I know only ever use GC seals to buy ventures and even then it is just as quick to farm leves and guildleves for them.