Opposition. Opposition forces. Competitors. These things strike me as being more what some are talking about -- people competing for resources or status or attempting to prevent someone else from having one or the other or both -- for whatever reason than what I think of when an antagonist is described as per the above. Characters in this group may behave in extreme ways against other characters, but it's as possible for them to all be acting from a 'good' motivation as it is for some desire to burn the whole world down or go full-on Mr. Burns. Some of the most amazing story conflicts can arise between two 'hero' types with different ideas about how to bring about world peace and happy ever after for everyone, for instance.
Generally speaking, I don't like the label antagonist for a number of the reasons described in the thread. It has nothing to do with worrying that someone is going to be mean to someone else IC, and more or less everything to do with the idea that there's a default set of traits that are acceptable for characters to have and one that 'others' them into a category of 'bad guy' that just doesn't work for me. It feels, on some level, as though the very messy, murky, muddy grey of reality in which people are made up of good and bad traits is not relevant on a MU, and that strikes me as pretty silly (for most settings; I can see some classic four color superhero settings going this route if that's the vibe they're going for).
The black and white breakdown of traits doesn't just impact your 'antagonists', either -- it limits your 'protagonists' as well by limiting the motivations, beliefs, practices, opinions, etc. that are open to them once it gets to a certain point.
There's a distinct difference between designated (or otherwise) antagonist characters and conflict RP. There's a difference, too, between conflict RP and actual risk, even.
I'm also with @Ghost re: reward for risk. I very firmly do not believe in tiered basic gains based on risk level for a variety of reasons. Most of all because risk involves activity, it's easy to reward that on an activity basis. Someone who is generally risk-averse may find that activity worthwhile; tiering it all presents an obstacle in that regard. Second, some generally risk-friendly players may consider some specific activity uninteresting; forcing their participation based on what's essentially an arbitrary ranking is, well, it's meh. It's just meh. Third... what's stopping someone from setting themselves to maximum risk and just sitting back on their butt and letting all the free extra XP roll right in? Absolutely nothing unless that reward for risk is tied to a requirement for activity.
This is one of those K.I.S.S. situations, and @Ghost nailed it.