After having played some MMOs, I personally think the combat gets dull because of two reasons: you end up spamming the same skill bar actions over and over, and you're put into an extremely specific role.
- As healer you primarily do one thing: Pay attention to tank and buff him and heal him as necessary, which mostly involves spamming the healing skills on him. Only if something goes wrong will you end up doing something differently and consider other party members.
- As tank, you have to aggro enemies, and let your party know when it's safe to attack. Once you're in position, you usually end up cycling through the same set of skills.
- As a DPS, it might be more interesting depending on your class, but a lot of the time it's about walking into range and then repeating some attacks skills until the mob is dead.
To me, this just seems extremely repetitive. And it has another problem too. Roles are are fairly simple but you have a huge reliance on them. Especially if the tank or healer does a mistake, things will go very wrong, and people will hate you. They will get very mad.
I played Guild Wars 1 for a long time, and I think it did some things to make those issues less of a problem. You had a lot of freedom with the class mixing and building up the skill bar. The bad thing is that most PUGs just wanted a pure effective DPS so they wouldn't be very interested if you were trying a new experimental build. But on the bright side you almost always had the choice of playing with the AI, letting you play in whatever style.
I think the most fun I've had with the trinity set up is when something goes wrong. Where the tank suddenly dies or loses aggro for instance. At that point you suddenly have to improvise and think on the spot, instead of continuing to spam the same attacks. But that has the downside that players will most likely get mad and angry, or if you're in a dungeon of high enough level, the party will just wipe instantly.
I'm personally excited about the current trend in Korea where many MMOs are going towards action based combat rather than traditional skill bar based gameplay centered around the holy trinity. Black Desert especially looks like an amazing game. I think that's the first MMO in ages I'll end up playing a lot.
It also looks furiously pretty. I'm half tempted to just keep posting screenshots of it: