1) Over 200 new NPCs: Prisoners that share jail cells. Most can be bought or stolen from them in their cells. Things obtained from them might aid you in your escapes.
Visiting castle nobles and diplomats that show up in castles. Hint: Nearly all are worth trying to steal or pickpocket from.
Inn travelers that actually use rooms found at Inns. They'll carry goods on them, eat, sleep, check their goods, etc. Great thieving opportunities.
Now the good stuff...
Expect a pretty severely revamped prison escape system. Used to simply picking a lock, running past a guard, grabbing your confiscated gear, and making your way to the town or city gates? Heh heh - "thems days be gone, brother...". The reality is that not only are the guards a little more numerous, powerful, and smarter, you'll also be avoiding traps, decoys, and ambushes on your way TO the exit points. If you can just run your way outside of prison in various cities while taking numerous arrow hits and sword strikes, hats off to you. I couldn't. I like my escapes to be tough and methodical.
All Castles are veritable sources of wealth now. Not only are the NPCs carrying more loot, there is more loot to be found. And none of it is overly obvious. Bring torches. Castles now also have more dim and slightly more atmospheric lighting. I'm a veteran hardcoe Thief fanatic from the old LGS days, so expect the same type and amount of dedication to the series.
Lockpicks are rarer and must be used more intelligently. Fences sell a few and you'll still find some on NPCs if you're pickpocketing them, but bringing 40 picks is gone. if you really run hard up for lockpicks, you can always try approaching Bandit thieves within their caves, camps, and lairs. Though they'll sell you some (if they have them, that is), you'll have to reach them first without being seen by the other bandits. Yikes...
Making your way to Fences to pawn off loot is no longer just a matter of finding them outside city walls. They'll have to be hunted down and sought out. Again, the first hints can be found within one of the secret lairs. And even when you hit paydirt and make your way to them, how can you be so sure it's really them and not the Watch impersonating them and trying to nab you? TIE veterans will know what I'm talking about here.
All hired guards now have full AI routines. They sleep, eat, etc.
Secret lairs have been improved and rendered more realistic. I'll let you find out which ones are on your own. The "key" to finding the first lair can be found within the tutorial.
A few precious books and scrolls have been added. Namely, a Mythic Dawn NOC (Non Official Cover) list. Want to eliminate baddies before they strike? Find this document and that chance might be yours. Good hunting.
Chances of catching diseases have been lessened.
And far too many smaller, less drastic changes and additions have been made to mention. All I can say is that at first glance, it will appear as though nothing has changed from 1.25. The differences are subtly implemented yet striking when encountered. Like I said, later this week. Perhaps Sunday or Monday.

Sidenote: Shivering Isles expansion pack IS required for this version of TIE. I will not remove any SIE elements from this simply because it is too integrated and I want to feel as though I've made good progress with SIE in the Shivering Isles (I have and those differences will be noticed to adepts). As always in my mods, I strive for compactness and a fully fleshed design structure as much as possible. Now that this version of TIE is near completion, I can focus on the Shivering Isles and aim for a November or December release. Of course, this all means I'll be updating my site to reflect the news soon.
Thank you.