Honestly, after playing some action MMORPGs now, I gotta say I'm an equal fan of both styles. Playing CO and NW has not made me want to "never go back again". I also think the slower games will always attract better quality communities, but that's just a hunch
I concur. I'm playing several games right now, and there's quite a mix of combat speed/thought. I enjoy both.
Comparatively slow games like CoH can indeed leave more room for thought and tactical sophistication (although in some cases, "traditional MMO combat" is just so absurdly easy that you don't really
need sophisticated tactics...SWTOR, I'm looking at
you). I'd place LotRO, GW, and most other traditional MMORPGs in this category. CoH did this style of combat better than any other game I've played.
On the other end of the spectrum, there's Defiance (an MMOTPS...). It's a
shooter (and not an imitation one, like some MMORPGs with a reticule option...I mean actual realtime collision detection and so forth), and the combat happens at that pace. It's not "thoughtful" in the sense of having complex powersets that allow synergies, contain a broad array of ways to ruin your enemy's day, etc. You equip two weapons plus grenades, and toggle between them. It is, however, quite "tactical" in the sense of you have to move at least a little bit in the direction of realism (taking cover from incoming fire, etc.) rather than just standing there and mashing the right series of buttons. It's quite fun.
Then there are "middle ground" games like The Secret World or Fallen Earth. Games like that (and I'm currently playing both...) have somewhat faster, more dynamic combat, but don't really slide into "twitch" territory. TSW requires good, smart (and sometimes fast) movement in combat against all but the easiest enemies. Keep moving or die, basically. It also requires finding good synergies within your currently-equipped "deck" (you can, with time, get every ability in the game and set up multiple "decks" that load with a couple clicks, out of combat). Some of the more difficult content (Elite and Nigthmare dungeons, Lairs, etc.) require timely use of impairs and interrupts, etc. It's somewhat faster paced than an old-school MMO, but not twitchy...and the combat can be quite challenging. Fallen Earth have a very well-implemented reticule targeting combat system, but the pace of combat is generally not that fast ("generally," in that sometimes everything goes pear-shaped and you really have to scramble). It's not a game in which a mad zerg tends to end any way other then very badly.
I think you're right about the connection between pace of combat gameplay and the nature of the game's community. Twitch games attract console kiddies...need I say more?