I'm put off getting 96 herblore by the cost. I'm 92 and it's getting really annoying trying to get lucky with stews. Another is runecrafting because of how bad the xp is. I haven't tried this runespan thing yet though.
Herb is a bitch to me, the time is nearly 20 seconds of MAX afking. Say for an invo of making pots takes about 20 seconds for all (14) pots to be created in the invo. Bank and repeat, the skill is actual hell.
Any skill where the xp is slow. If I'm slaving away at a skill getting like 50k/hr xp, when all I can do is think of how much more xp I could be getting doing another skill.