Huh, so from most of my friends, and indeed, the internet at large, it appears Thief is a mediocre-to-bad game. I should clarify, none of my friends have actually played the game, but the reviews (presumably critical and user alike) have firmly implanted a bad taste in everyone's mind. Which I personally think is a shame. Thief may not be the greatest game of all time, but it is a good game, only occasionally tarnished by sometimes buggy controls, and odd level design.
In fact, I have enjoyed it so much that I have invested in excess of 60 hours on my first playthrough (yes, I am an obsessive collection player). From the get-go I cranked the difficulty up to Master, playing through ghost-style (so long as loot collecting permits), no kills, no takedowns, sans Focus - and it's challenging and fun!
Now a little more detail. Thief boasts a nice slick look and feel, from its steam punk aesthetic to polished assets, down to the detailed animations when you pick up or pilfer items. I especially love pick pocketing. The many mini-games, eg lock picking, pick pocketing, setting combinations, built into the many larger puzzle elements, eg level navigation, general sleuthing, makes a solid basis for good gameplay.
The plot is new, with only incidental references to its predecessors (Garrett's mechanical eye, odd mentions of the Trickster), and delivers a contemporary story line. It doesn't have the same pizzazz as the original of course ("they took my eye!") but few titles since then ever have.
Those are certainly my highlights from the game. The things I didn't care for, and can't ignore here, are things like "swoop" being stuck on - such that movement in any direction suddenly results in a swoop. Very tiresome when you are trying to sneak up on someone, or making a precision movement in a scene. Fortunately, this behaviour was rare, but it really soured a good hour of gaming. I am also puzzled by The City's odd divisions. I presume it's due to the game's troubled past, but also the many platforms it has been released to, including older 360 and PS3 platforms. As well, not a criticism of Thief alone but all Unreal titles: I am continually dismayed by the total lack of progressive loading. Load screens kill games.
Anyway, I am really enjoying Thief. I have described it as having done for the original Thief what Deus Ex: Human Revolution did for its predecessors, for better or worse. I think Syndicate also enjoyed some notoriety for its departure from canon, and was also a good game in and of itself.
Either way, I would recommend Thief, especially if you enjoyed Deus Ex: Human Revolution. Thief exhibits the same care and polish, and has a lot to offer of its own.
Give it a try!