imo DB/aAa/GIANTS! were the weakest teams and Wolves deserved to keep their spot. The Wolves are the only ones that are capable of breaking the top 4 at this point. Both MYM and aTn have tournament experiences and have a ton of potential, while the newer SNS has both Tabzz and NukeDuck, two very strong players. I would not be surprised if Wolves or even one of the new teams manages to break top 4 at some point, likely kicking EG or SK out.
NA has some scary newcomers, too. coL and MRN are the weakest teams, and are probably going to face Quantic and Velocity, two teams that are both REALLY good. Quantic (which has most of the Cloud 9 roster) has a lot of experience, as does Velocity.
Dignitas's bracket is pretty easy, as I'm not expecting much out of Team Summon or Fidelis. CLG, on the other hand, will have to face one of the best teams in the entire promotion series. DoubleBuff (formerly One Trick Ponies) has a lot of potential, while Azure Cats is Jiji's team and also has Cruzer on top lane, Demunlul on ADC, and some other big names to round out the roster. I don't see Dignitas losing their spot in the LCS, but CLG could realistically get knocked out. I'm also predicting that coL and MRN will be knocked out by Quantic and Velocity. In EU, none of the ranked 5s qualifier teams made it past the challenger round. I'm predicting the same for NA, with the exception of Jiji's team, who I think will be able to defeat DoubleBuff.
Obbmud: You need a Last Whisper in that build. Usually, by the time you finish your first major damage item and your first major attack speed item, the enemy team will have some armor and you'll need the penetration. IE-PD-LW is pretty standard on most AD carries, and for good reason. After that, BT and BORK are the two best lifesteal items, and you'll usually need to choose one. For their last item, most players will choose to buy a defensive item such as Randuin's, Warmog's, Banshee's Veil, QSS, or Guardian Angel. If you want to go full glass cannon, you could definitely build a second PD (Shiv probably not recommended as a last item because PD outdamages it when you're at 6-item status. If you got a Shiv early, that's fine) to maximize DPS, but you'll be losing out on a lot of safety.