I mean it is very probable that what the OSRS gold matter is that the responsiveness is population. If you are utilised to it being 0.6 seconds per tick a very minor deviation from that will cause massive issues. That is something people do not think about when they say"get rid of tick system" or even the more educated"make quicker ticks" - should they triple the tick rate we will get much more population based tick speed fluctuations. Unless they buy servers that are much too much for conducting RuneScape 99% of the time. It comes down to money, 99 percent of the time. Thus from jagex's standpoint it is not an issue, because fixing that will be throwing away 99 percent of the cash used to repair it.
I do not really consider it the biggest, although tick rate is the most fundamental issue. The biggest issues, I think, are at the core gameplay: specifically the controllers (both movement and battle ). Click-to-move (and by extension, the tile system) is my biggest gripe with RuneScape's heart gameplay.
It makes motion sense clunky, and because both motion and targeting are done via the mouse, so it creates moving through enemies and precise positioning more bothersome than it otherwise could be, and normally makes battle less fluid and more intuitive than I'd need it to be. Now, WASD move on a tile-less gameworld is a pipe-dream for RS3, but I would be remiss if I didn't bring it up as my largest gripe.
That is one of those things that simply won't change because if you eliminate the click on Cheap Runescape gold and point facet you remove one of the simplest elements of RuneScape. I think that it would be helpful to have WASD instead, but by eliminating click movement 18, people would be pissed off.