Yeah that game is really cool when you get into it.
Ive played both Thief 1 and 2 a number of times through.
There was LOT of fan made missions at;
http://thiefmissions.com/ Only recent fanmade missions are listed. Read why yourself.
ALSO
http://www.thief-thecircle.com/ Which has a link to the Unofficial Thief 2 Expansion. Its a
very spooky update and well made. Also has a lot of fanmade missions
for download. Click the first link on top. When searching for missions, where it says 'Sorting Method' choose 'Hammer Score' and you will get the best missions listed first.
AND
http://www.ttlg.com/ Fansite for Thief, System Shock, DX and Arx Fatalis.
As for those trees I freaked out a few times when they chased me
as all I heard was the scream and then THUD THUD THUD as they
ran toward me.
I was like WTF!! was that.