Although I know it's been said, personally I find that The Answer Lies Within and I Walk Beside You really don't hold much ground against many of Dream Theater's other lighter and shorter songs in their catalogue (both prior to and after the album was released). The Root of All Evil and Panic Attack, while good, are very standard mid-length Dream Theater songs with very little to differentiate them from other mid-length songs. Sacrificed Sons and Octavarium both drag on in the first half or so and feel somewhat inflated (again, this is my personal opinion) and Never Enough is widely seen as borrowing too heavily from Muse's style.
So yeah, on the whole Octavarium comes across as weaker than Dream Theater's other albums, and as such, it is my least favourite album that they have released. That isn't to say that it doesn't have its strong and even amazing moments, only that there are fewer of them than Dream Theater's other outputs.