Hearthstone cooldown halved: The cooldown on your hearthstone will be reduced from 60 minutes to 30 minutes. I think this is particularly convenient for short play sessions or times when you have to end your playsession abruptly due to real life interruptions. Just hearth to an inn and exit the game, guaranteeing rest and relative safety. But it's also good for questing around hubs for longer playsessions.
Ground mounts will be able to swim: No more being dismounted when you hit deep water -- your ground mount will be able to carry you to shore. This is a nice little convenience for questing and just travel in general. Be aware, however, that flying mounts will still dismount you if you try to swim.
You will be able to queue for Battlegrounds from anywhere: Rather than going to major cities to queue for BGs, you will be able to queue from anywhere in the world and when the BG ends, you will return to where you were. This is really great for those of us who PvP and have limited playtime. Now we can much more easily fit both questing and Battlegrounds into our playsessions.
Dual Specs: The most highly anticipated feature for Patch 3.1 is the ability to toggle between two talent specs on the fly. While this should make things easier for raiders, I think that many casual players will find this new functionality very convenient as well. If you are level 40 or over and have the spare cash (a one time fee of 1000 gold at this writing), you can pay a trainer to give you a Primary and Secondary talent setup to choose from. I think this will be very useful for players who PvP so that you can have a questing spec as well as a Battleground spec. If you are able to do 5 man instances, this will also allow you to have a solo spec and a group spec. This will also allow you to have a tried and true spec and an experimental spec so that you can try out different specs in your secondary setup, while still keeping your known-good spec. Dual Specs will allow a flexibility that I will definitely be making use of... once I save up the cash for my favorite characters.
Built-in Talent Calculator: This is functionality that I think should have been implemented at launch. In 3.1, you will choose your talents and then accept them, rather than spending your talent points as you go. I can't tell you how many times I've clicked incorrectly and literally had to pay for my mistakes. Now you can fine tune your talent choices in game before being stuck with them until you respec. I thank you, Blizzard, and my virtual pocketbook thanks you.
New Quest functions: Now you can right click directly onto the quests you are tracking to do some quest management as opposed to having to go into the Quest Log to do everything. It's a nice little feature that makes the in-game quest tracker more convenient but doesn't replace Questhelper or any other addon like it.
Gear Manager: Last time, I talked about addons for casual players and I recommended Outfitter. Outfitter is a gear management addon which allows you to setup different gear sets to be used in different situations. It has the added functionality of automatically changing to sets when certain conditions arise. For example, it will change to your riding set when you get on your mount. Or, for druids, it will change to the appropriate set when you change form. The new WoW Gear Manager does not do any automatic wardrobe changes. You save gear sets to buttons that can be hotkeyed. The other function missing from the in-game Gear Manager is extra storage for gear. When this feature was first discussed at BlizzCon 2007, the developers bragged that they would be able to incorporate more bag space for your gear into the functionality of an in-game gear manager. Unfortunately, this has yet to be implemented. Overall, I see no reason to stop using either Outfitter or Itemrack once Patch 3.1 is released.
There are a lot of small changes like confirmation boxes on purchases over 150 gold and I am sure the patch will continue to evolve while it is being tested. Features are, of course, subject to change without notice. But as things stand right now, it looks like WoW will get more convenient for those of us with limited playtime.