As I sad, personal preference.
I am host of an small GT-racing community in Croatia, we have like 100+ dedicated members.Of all of us, maybe less than 10% actually cares about leaderboards because majority of us just spend all the time online in MP racing, championships and such. At this point, our mutual crave goes for introduction of more tracks at some point and it is the only improvement we all want unanimously.
Few of our members are hard-core Time-Trial guys who would like to see full-leaderboards but even current format of Seasonal Time Trials suffices to them because they all pretty much take the TOP10 places this way or another and introduction of full-leaderboards would just dissolve their focus in conquering the world with Croatian flags
In short, your personal preference would be leaderboards.
My personal preference would be online racing with forced stock settings or forced particular car on stock settings where no crap such as PP limiters, crazy suspension settings adjustments and such would ruin the competitiveness.
For some other guy Livery Editor is a must, while he does not care about leaderboards or stock-settings options.
Fourth guy would like to see more A-Spec events because he can't go online and enjoy great Seasonal races, or he just don't see more than 40 races now available in Seasonals as extension of A-Spec mode.
Fifth guy can't stand existing B-Spec format because it does not allow him to fast-forward the carrer in his goal to Platinum or credits, while sixth guy plays only B-Spec, favors all improvements made in B-Spec form GT4 and finally have a great feel of actual accomplishment once he beats the AI in full real-time RPG racing that exists only in GT5.
It all comes down to personal preference, really.