Rainbow 6 does have the huge green markers over your allies heads plus the combat is pretty much always under 100 yards so spotting people is never an issue except for dark areas/smoke/flashbang which are purposely designed to limit visibility.
My point, which Silent_Buddha so helpfully illustrated, is that you can't bring fun/not fun into this discussion: people will like different things. Saying not instantly recognising an enemy is not fun is ignoring the fact what each individual human finds "fun" varies.
Also, notice how Grall, who doesn't like shadows in MP games apparently because it makes it harder to see enemies, wouldn't like Rainbow Six which went on to spawn eight sequels.
If you're talking about a game where there's essentially an open world, having map markers or some such thing does definitely reduce how often you get killed by players you never saw.
We're only discussing Brink and similar shooters. By all accounts Brink is a small scale MP game sporting 8v8.
I am not saying they need to be neon green and neon red Games like 1942 had uniforms of similar color but design choices made it fairly easy to tell them apart. Games with a lot of post prcessing in dark settings can be hard to see stuff so some artistic considerations can help. I am not playing the game, so as an observer sometimes you are not always as engaged, but it seems guys looked similar?
Well, ETQW had some of the most instantly recognisable models of any game for the two teams (short of say, Humas vs Spiders). The human team was your standard man with army fatigues while the Strogg were metal-based with reasonably different silhouettes: SD still put in red triangles over enemies' heads.
They did leave in a console option for disabling these red triangles but two things happen: you had to be running a custom server so the only way to play on these servers was search for servers with the "modded" option checked which usually meant joining a server running an actual third-party mod... which isn't always wanted.
Secondly, because it was a modded game it wasn't ranked. Most people only play on ranked because they measure their worth in life through gaming online stats like how many times they've tea-bagged someone. This meant modded servers were usually empty or nearly so: ETQW's sweet spot is 24 players (that's what ranked servers run in).
Like I mentioned, SD have already said they're leaving out the red triangles in Brink. Let's see if they carry this through.