For -me- it is the training plateaus we get on most skills (not all.) I have had MONTHS trying to get gains on some skills (in the first year there was very little factual evidence as to what worked best.) Even now I sometimes will have a few days of real play before I get a gain, largely for trying to find appropriate level monsters to deal with. For lumberjacking, it was just finding the time to mindlessly chop trees since I knew I'd get a gain eventually.
Now that we have play guides online that give us better ideas what to do at what levels, the heavy pain is abated. However, there are some skills that just very very tough to work on to get consistent gains at the higher levels.
I'm one of those that prefers GAME PLAY and not boring training. If I'm out and about in the world fighting and doing things, I really should have a much higher chance of getting gains than I would at Luna bank or in my house.
This should be a natural effect of wandering the world and going into dungeons. While scrolls of alacrity do this function somewhat, I think
everyone should have that natural bump to gain for actually playing and not sitting in their house casting earthquake or beating up golems in Luna.
As for the training -- the devs don't always seem to understand that people don't like taking a low skill character out to play with the big boys. It's not fun to take on a monster with your current "best" and get killed in two seconds. Believe me, I remember doing that and I HATED trying to find a healer, hoping I had most of my things when I got back to my body, trying to avoid other monsters. That also happened when I played with others.
"Well don't go fight things you can't take on". Completely idiotic argument. Yeah, I keep a notated list of every monster and skill level so I know what not to fight as a new player. Even as an old player I sometimes am surprised by a monster being more dangerous than I thought (paragons sometimes surprise me, as do town invasion monsters.)
You have to get players interested in taking their characters out -- but many of the invasions and activities tend to be geared more towards those that have plenty of experience and ability.
Perhaps the higher difference a monster has between it and your abilities, the less interested it would be in fighting you. We already have a "marker" -- barding difficulty. Why not have monster aggression check vs barding difficulty and "decide" whether it wants to try to attack you? This would be far more realistic anyway -- why the heck do orcs attack my near legendary fighter, that just took out a dozen of their mates?
Anyway, sorry to go in a tangent