2) Armies aren't equally represented, Imp Guard are expensive, time consuming, and bulky these tend to be factors that lower the number of people willing to play with them (It also seems like a lot of people got bored with that dex). Crons are affected because the whole range still isn't available. I think there were no Nid armies at adepticon at least in part because people didn't have to time to get the new tervigons built and painted (or rebase, and make theirs bigger in addition to all the points about IG) not suggesting they're good though. Dark Eldar are IMO under represented at least partially because they're unforgiving, and also because you don't know what to expect terrain wise from tournies and their codex is designed around having ample cover. My earlier point about TLoS really applies to them in a big way, their transports are just so fragile that even a 4+ cover save doesn't help too much but with aerial assault and open topped they would gain alot from the old area terrain rules.
3) Blood Angels have a lot of "traps" I think they're right up their with wolves & IG, but its easier to lose focus in that codex, and the jumper list is a lot of fun but has some horrible matchups which I think drags down their results.