Bloat occurs when a forum software tries to do "everything" built-in, even when the features are very niche.
An argument could be made that vBulletin's calendar and social groups were "bloat". Those features are not critical to
every single forum in existence. They are critical to
some communities, but not
every community. For that reason, these features are best left to add-ons (be they official or 3rd party).
That's only part of the problem. The real problem is when niche communities get a hold of these features and start trying to use them for their particular niches. Take a calendar feature for instance. Core features would be, per-usergroup ability to create an event, member birthdays displayed in-line with an option to click to see the member's age. Those are pretty standard features for a calendar feature. Seems reasonable.
Now let's say a niche community starts trying to use it.
Is it a community for real-world meetups such as conventions? If so, the calendar needs an event system that allows people to RSVP with "Going", "Interested", "Not Going".
Is it a community for people who are struggling to overcome addictions? If so, they do not want nor need RSVP functionality as the events added to the calendar will be used to mark milestones for sobriety.
Already, you have a huge chunk of functionality (RSVP) that is completely useless to an entire part of the market that may have uses for a calendar. But we're not done yet.
The addiction recovery community needs recurring events, since (in an ideal world) the members want to mark their milestones automatically ("Another year sober", repeating yearly). The convention community does not, since their conventions have to be planned and booked before the event can be added, it is impossible to guarantee the same convention occurs on the same date in the same location every year.
That's another thing, location. The convention community wants to geotag their events, with a Google Maps pin. The addiction recovery forum obviously has no need for this, since I would imagine most of their members do not want to give away the precise location of their homes.
Are you beginning to see how what seemed like a simple calendar feature has rapidly diverged into two entirely distinct feature sets?
Now, let's assume for the sake of argument the differences end there (they don't). Let's say you are trying to set up an addiction recovery forum, because your group is growing and while you want to be there for all your group members when they need it, you also like sleeping, so why not set up a forum so that people can get support even when you're asleep. You are an inexperienced (or entirely new) website administrator, so having 500 bajillion features would be very overwhelming. You want something nice and simple.
Putting yourself in the shoes of someone who, like Jon Snow, knows nothing: Can you say that having to check to make sure you've disabled RSVP and geotagging and whatever else for several user groups is a very simple and straightforward task? Because if you're trying to tell me that it is, then I am going to call you a liar.
The mind control drug I put in your tap water is working, because I know what you're gonna say: "Why not include a basic calendar feature in the core and let 3rd parties add the advanced features?"
Because I can think of at least two members from this very community that would be all over the suggestions forum like a rash with a 30 point list of features that are desperately needed, noting that "why even bother adding a calendar feature if these 30 points aren't included?"
If you add a calendar feature to the core, then it simultaneously needs to do everything and only the basics, and it needs to intelligently infer which feature set to present to the user based on magic and/or more mind control drugs in the tap water. One community will see all the RSVP and geotagging features as needed & standard, another community will see those features as bloat.
One community's bloat is another community's standard required features. There is no way to win.
Fillip
