Aaaah, thief.
I remeber the first time I just tried the demo on a slow computer and I couldn't play it well, so after the first death, I just gave up.
Later, when i got a better computer, without anything new to play, i decided to install some demos from a lot of magazine CDs i had. Then thief demo would run properly and so I loved the game.
I had just graduated and with my first salary I bought thief gold and thief 2 metal age.
That's why I can nevere really classify which one is better, since I played the first game and jumped right to the second, and to me, it just feel like they're just one big game.
And thief 3 has only engine limitations against it and some bad gameplay decisions, but other than that it's the same game and has the same feel of the originals.
there are some mods that make the 3rd more like the originals, but with minimalist mod you have to be careful that it raises the difficulty level to a different way than the originals. But that can be changed in the .ini file, so you still have the "deconsolization" and original difficulties levels.
One thing: I AWAYS played on expert, even on my first playthrough. Difficulty in thief only imposes limitations, like no killing, get more loot. I believe the AI is the same, damage is the same. On expert, you have the true experience of being a thief. It's the proper way to play, and it's not that difficult. And yet, you could technically "kill", since blackjacking an enemy would just invalidade the guy for the whole level. So, you really don't lack any options.
There's just one level in which you can't even blackjacking anyone, but it's on thief 2. And it's great that way.
Other advice: some one said upthere that turning off your lights is optional. In fact, it's not. Turn it off aways. Some levels are very dark, and a lit room may force you to raise your gamma, which breaks atmosphere and feel of the game.
EDIT:
About mods, the enhancement pack only rises the poligons count of jars, cups, chairs, and with installed or not, you hardly notice. I believe some new mods required then to be installed, but some already come with it.
I'd recomend the AI meshes from thief 2 to be installed over thief 1, though. I doesn't change a lot, but it's better on the eyes for someone whom never played.
Going to the difficulty problem: Well, I think someone new can be bored with the game if they play killing everybody. You finish levels faster, get to the objectives, but will miss a lot. You won't feel like a badass thief you're suppose to be.
And yet, you can blackjack everyone on expert, save for some thief 2 missons. And by the time you reach there, you're probably good on the game already.
You can even backstab zombies. Oh, and you CAN kill zombies, since they're already dead. Playing on expert I never had problems with ammo and items. You start the level with enough ammo and items, you can find more on the level, plus you can buy more before entering the mission.
I see no point playing on easier difficulties, most of the surprises and awsome moments happen when on expert.
I had once let an uncouscious body fall underwater in cragscleft prison. The guy died 1 minute afterwards and game over. I didn't realized that he felll on the water, and I was amazed the game recongnized this. If it was on diffcult, the guy would die and I wouldn't notice.
About a new thief or thief clones:
I think the first 3 splinter cells are the closest clone of thief we ever had. Splinter cell was more forgiving, but gameplay mechanics was about the same. It's funny that I played conviction recently, and finish the whole game in ONE day. At the same time, I was replaying splinter cell 1 and even knowing all the background, what to do, it took me two weeks to finish the game. I repleyed conviction and finished it again before that playthrough of SC1.
I believe that if thiaf follows this trend, we're doomed.