Demonologist Build Monotony
Currently building a Demonologist is kind of boring because you really don't have a lot of choice with how you build your character if you want to be effective.
Both PvP and PvE builds benefit the most from having a split between the two main trees, with around 30 points in Conflagration and the rest in Havoc.
The demo is a very effective class by all accounts, but the fact that it's so one dimensional in what makes a good build also makes it a pretty boring class to customize.
The reason why there is really no good reason to specialize your build with a demo is twofold:
For one, the top tier feats in both trees are simply not as powerful as the lower tier feats. When you get to the high end of the feat trees you don't find anything there that you can actually use all the time that brings a specialist in one tree up to par with someone who dabbles in both.
Secondly, the general tree for Mages is not particularly powerful when compared to what you get out of investing 30 points into the bottom of the class trees. You need to get to the Third and Fourth tier of the general tree to even start seeing feats that are really worth the effort, and there is nothing that significantly buffs your spells.
One of the main reasons why things play out this way right now is that Shock and Hellfire Stream as well as Shockblast and Inferno of Ahmer are both at their best when used together. Since those spells pretty much always have to go together to fill their role in the best way, be it mobile damage or massive AoE you simply can't get around having to max out both spells for the best results. Neither the Havoc nor the Conflagration tree offers anything at the high end of it that curbs that dependency on the other tree, and as a result heavily specializing in either tree simply isn't as fruitful as spreading yourself between them to get the most oomph out of the spells in both that always go together.
I think this class would be a lot more fun if going all the way in either tree actually allowed you to make a good character. The top tier feats need to be more worthwhile, and most importantly the top of the tree should contain powerful feats that allow you to not be dependent on the other tree anymore for a major chunk of your damage output, and thereby always bias the class toward being a hybrid build.