Vertical progression is not necessary bad but it's the amount of content and the 3 month patch cycles.
Every 6 months we get a new 8 man raid that has to have the best gear
Every 6 months we get higher tier tome gear that have to be just slightly less good as the 8 man (savage) gear.
Every 6 months we get a new 24 man that has to have worse gear than the current 8 man (savage).
Every 3 months we get new 'experts' that has to have gear that's not as good as 8 man (savage).
Now we have the diadem that gives 210 gear with very little effort. This was acknowledged as a mistake though.
There's too many ways to gear with very little manoeuvrability with ilevels. Within 3 months the expert can eclipse the 24 man gear.
There needs to be a better plan with regard to gearing and the three month patch cycle.