A lot of classic scams are gonna be back. Thankfully, with the big exclamation points and the wealth transfer information on the bottom of the trade window, there are a significant number of deterrents now. But some items were definitely tricky and easy to scam with. Noted mithril/rune items, and Guthan's spear/leaf bladed spear come to mind.
Just a few scams I remember that may still work:
-A two person scam where one person is selling an item for more than its value, and another one is buying it for even higher. Player buys it from the seller, both of the scammers log off.
-Set up a legitmate trade, "accidentally" cancel, and retrade with a lower value item. People used to try this with me with trimmed armour. Ex.: At one point addy(g) bodies were 300k. I'd put up the coins, and they'd put up the platebody. I'd hit accept, and they'd "accidentally" hit cancel, and retrade. I put up the 300k again, and they put up a regular adamant platebody and hit accept. Thankfully I learned to always check the screen carefully, but I be this one got a number of people.
-Noted mithril items exchanged for noted rune items. A good number of noted items, like mithril/rune picks, mithril/rune hatchets, and to a lesser extent armor, look similar.
-Trust games (for the really new players)
-Drop trades (same as above)
Also, the wilderness is gonna be a much more dangerous place (obvious). Slayer tasks like Green dragons and distractions like treasure trails are gonna become much more difficult.
On the plus side, it will be easier to give gifts to friends, share expensive pots with teammates, and of course there were some really epic drop parties!