Great interfaces, NWN2 & WOW. Very configurable, lots of hotkeys, with a little work you rarely have to go into a menu.
While totally fluff, it helps to surround your UI with a little graphic to tie the interface to the mood of the game. Arcanum & WOW are great examples but NWN2 didn't even try and it distracted from the ambiance.
Keyboard shortcutted squares that you could drag anything (ANYTHING!) to. A weapon to switch to, a spell, a skill, etc. I liked how NWN 1&2 allowed you to put 2 weapons in the same square for dual wielding or sword & board. And those were just shortcuts, so the shortcut didn't go away ( unlike Arcanum where you put the actual item in the shortcut and then the shortcut was gone after use ).
Every actionable UI item should have a mouseover description. Every hotkey square should be context sensitive ( ie if there's a spell in that slot, show the mouseover for that spell, if it's a weapon, the weapon mouseover ).
Drag & drop interface, especially in inventory, right click is auto-equip or use. Right click quick buys/sells if at a vendor.
Since it's turn based it's easy to separate the combat UI from the exploring UI. Take advantage of that. Keep the exploring UI minimal but don't be afraid to fill out the combat UI, one of the problems of turn based combat is the length of time it takes combat. Making spells, skills etc as fast as possible to execute helps with the speed and tedium.
Think about right vs left click. Every button has a normal left click but take advantage of having two mouse buttons, it's appalling how few games do this, yet I've never seen a PC gamer with a 1 button mouse. Example: A spellbook button, left click opens the book, perhaps right click opens a quick list of all spells in a simple menu for speedy access. If something has two steps to execute, for example perhaps casting a spell requires you to choosing how much mana to spend to cast it, left click is normal brings up the how much power dialog but right click always casts it at full power.
If something has a scroll bar, make sure the wheel works if my mouse is inside that area. It's sad how many developers working on console ports don't bother to make the mousewheel work anymore.
Keyboard shortcut everything that makes sense, the hotkey squares and major actions ( inventory, spellbook, character sheet, etc), and the end turn button at a minimum.
This should go without saying but use a readable font ( BG2, Oblivion & Return to Krondor I'm looking at you ). Scale it for PCs, not for TVs 15 feet away.
A lot of gamers are using 7+ button mice these days, no need to have default configs for the extra buttons, but make sure they're mappable. A lot of gamers also have Logitech G-series keyboards with the LCDs, it's a nice feature if you can support those.
Bad interfaces, that's easy, almost everything made in the last 5 years that has a console port. Alpha Protocol, bad. Borderlands, really bad. Menu based interfaces suck on a PC. Super bad are direct console ports like ( going a ways back here ) FF7, where they didn't even try to PC the interface, they just mapped the arrow keys to the controller d-pad.
Things that aren't so great in "good" PC interfaces. Inventories that have pages (NWN 1&2) or excessive scrolling ( Wiz 8 ). Personal taste I really like my inventory items to have relative size (Diablo, BaK & NWN1) instead of same size tiny icons for everything ( NWN2 & Drakensang). It really helps give character to the items, makes them more unique, it's more exciting to get that item.
Don't ask me a confirmation when I click something; I just clicked it, yes I want to do it (possible exception, quitting without saving ).