1. I hear that very hard to spectate Sacrifice
2. And also, ID ask players what game mode they want to see

1. Someone wrote already here that ID in future will add minimap on screen(like in CS). But still there is no minimap.
And during QuakeCon i saw not enough cameras positions for common locations.
Also i suggest to add 3rd person of view ability when you spectate.
Also, will be nice for spectator, if "common view" cameras can concentrate on some player(can change her angle of view) (player body on center of viewer monitor, always) and camera automatically switches to next camera if player behind the wall.
Will be nice if monitor space can show 4 players at once(multipov), or minimap+player(3rd person of view or 1st or autocamera of common view)
So, spectator can resize minimap(to fullscreen or to 1/4 of screen ..etc) like in CS, can change her transparency.

Minimap shows items, players, their view angles, may be their abilities and flying ammo's, hp etc.

All i mentioned now, already realized in some games. But also i think about Quake movies, where we have "smart camera" from moviemaker. So, if ID can create automated "smart camera" algorithms, we can get auto moviemaker and watch games like films (slow mo, auto replays of some hot moments during game or pause, etc).

2. I think CA very simple for newcomers. And very 1st candidate for LAN tournaments as main mode. Not sure about CTF, because it's like Sacrifice and more difficult for first Acquaintance. I dont know about what team game mode more popular were in QL. I think CA, because it's more like CS ;) But as old q3 player i prefer CTF or Sacrifice for spectating.