While I've not played it. Whenever someone makes that claim I get this feeling that playing it now I would find it terribly antiquated.
Getting back in shape for the reboot?
Damn right I am. Going to be off the face of the planet from the 25th until I beat the game on an ironman ghost no-spotted run. (well, maybe not that masochistic)
And, playing Thief 2 now is an excercise in patience; the game did not age poorly, but it is showing its age; its main draw is that the AI is remarkably savvy, all things considered (like the fact that it's 13 years old and its AI can run goddamn rings around more recent games).
And Thief 2 is the best for three reasons. Reason one; Shipping... And Recieving. You're given a block of warehouses, a handful of guards, and the tools of your trade, and told to shove off and start looting the place dry.
Reason 2; First City Bank and Trust. The only time in the original trilogy that you heist from a bank, the difference between sneaking about in a middle-class neighbourhood, or making rent by pilfering the dock warehouses and being driven paranoid because the upper-class bank is basically lined with marble, making it nearly impossible to not make noise, adds a great many tension-filled moments. But, it's so rewarding as the bank vault swings open for your efforts.
Reason 3; Life of the Party; Rooftop running, Tower-exploring, thief-thieving, and more, and that's Before you get to Angelwatch, Central base of the mechanists. But, Karras knew you'd be coming, and left a surprise...
In all seriousness, Life of the party is one of the largest maps in the thief series, one of the most-involved, and complex areas to explore and loot, and goddamn memorable because, after easily an hour snooping about, finding the evidence you need (and padding your pockets with lucre), you find your objective, and get a nasty surprise. As Shalebridge Cradle is what causes veterans of the trilogy to feel genuinely unnerved, Life of the Party will almost always have fond memories of the shenanigans you could pull.
The biggest draw of the Thief series in general, which was actually carried over into its nearest cousin, Dishonored, was that there's a Massive amount of worldbuilding done, and shown, be it guards bitching about how Bears now are too piddly and weak, and need blades strapped to their paws and muzzles to be a challenge at the bear pits, or two commoners complaining that since the mechanists began to rise to power, growing fresh fruits or vegetables in the city was becoming nearly impossible, and that they missed seeing trees along the streets instead of mechanist lights. It all adds to the feeling that The City is a real place, giving it the depth to make it have a lasting impression.
That is why I love the Original Trilogy, and what I hope the new game will carry forth.