The thing that I've been mulling over is how GW has workes out their tiers. At first I thought that maybe Tier 1 teams were all ones with cheap re-rolls but Lizardman shot that down (still not sure how that teams fails to work together well enough to get cheap rerolla considering the whole Lizardman class structure but it's probably for game balance and not fluff).
My best guess on the GW tiers are:
Tier 1 teams are built to do what you want them to do most of the time (excluding bad plays or Nuffle of course).
Tier 2 teams are built to do what you want them to do some of the time (by having lopsided team designs, expensive re-rolls, a lack of skills or skills that aren't as strong as stuff like block, dodge and sure hands which are more common on tier 1 teams).
Tier 3 teams are built to do what you want them to do a lot more rarely. These are the stunty teams with their whole design being built around making you roll as many dice as possible to do almost everything in a game where not rolling dice is key.
Now obviously we can argue the finer details, but this is how I think GW defined their tiers and I can agree with most of the team placement that way, but I feel that some teams nearly exceed their tiers due to skill and stat combinations, while others struggle to stay in them due to poor stat and rule combinations (like giving OWA loner -and- animosity. If anything they should have just made the re-rolls more expensive over loner and dropped animosity all together since they're a team that's supposed to be working well together unlike a team.like Orcs who squabble now that the Black Orcs aren't keep in them in line).