Well i played the original Thief & Thief 2 and 3. I can see a few flaws or rather "dumbing down" , but i did not find this game bad at all. It has the atmosphere nailed down, it has the whole thief-talking-to-himself thing and all that. It's clearly based on Thief 3 rather than Thief 2 though. It was in the Thief 3 where they started with this open city exploration thing, which i think did detract from the core gameplay, so likewise it is not as focused in this latest iteration.
Thief 1 & 2 , was like the old Rainbow Six games where you kinda have a lobby and then go on missions. Each mission was then quite complex and intricate, as in you are presented with a big mansion and have to crawl around and do various objectives while also figuring out more about the plot. This could take like an hour to do in some cases. So while the missions are arranged in a linear fashion (have to complete the one after the other), the actual mission itself was a complete open sandbox with options.
In Thief 3 and this Thief, i felt it's a bit more all over the place where they replaced the lobby with an open city to wander around in (which also sadly ads unneccsary travel time to everything, in GTA this is fun as you are driving a car, sneaking back and forth constantly to talk to various quest givers, not as much fun) . This results in small missions or missions that are broken down in phases, you don't quite get to know a specific area intricately enough to want to try various approaches. It is actually alot more linear, while the game make you THINK you got options/freedom (similar to Assassin's Creed with the assassinations, you can go do them in any sequence and fiddle around with side quests, but ultimately you need to do them all).
Overall though, it's still not a bad game, i think it's getting unnecessary bad flack because it is not living up to Thief 2, while it is certainly better than Thief 3 (for me anyway).
