I think the trading opportunities features are the largest area this site could expand and improve upon.
If there was a way to make them searchable with numerous filters in the same fashion as the card database, that would rule. Some search features that I think would rock include:
1) Last online (as HikingStick points out)
2) Feedback rating - if you want to avoid people with zero (or very low) feedback
3) Local vs Mail trading - how many times have you found someone that has the cards you're looking for, but they're set to local only? I have plenty of times.
4) Some sort of value and quantity sorting - I'd love to be able to search for people who have a certain number of a particular card or a total value from your tradelist (making it easier to unload bigger things if you're looking to trade down)
5) More ability to identify overlap between tradelist and wishlist - for instance, being able to search for people looking for a particular card that have items in your wishlist.
These are just ideas and we're all just dying to see what Sebi has in store for us with the forthcoming update, but all of these would be awesome.