From a player versus environment (PvE) and player versus player (PvP) standpoint, this means more players to group with directly. Activities like raids and guild recruitment won't require nearly as much realm transferring as before. Another effect is Rated Battleground groups and arena groups can form more readily. There is talk that symbol will be at the end of a name, like the asterisk (*) following a cross-realm player's name. Because of the physical separation and load balancing required to keep a realm running smooth, Blizzard is avoiding physically combining realms onto the same hardware. There are also financial reasons behind the decision, but I think it's best to just avoid that topic all together, for now.
Now, for my preparations I have to mention the World of Warcraft auction house changes. With the virtual realms comes a single auction house for each faction that combines all of the auction houses of each realm into a monolithic behemoth place of commerce.
For example, you are on the realm Echo Isles Alliance and you see maybe a few dozen armor pieces up on the auction house. After patch 5.4, your realm was combined with Stormrage. All of the sudden, you are looking at thousands of armor pieces to choose from for your sets. Now those Bind on Equip (BoE) armor pieces you collected while doing transmogrification runs are suddenly quite valuable.
This is the situation I'm preparing to handle. Several of the realms I've collected armor on are very low population, thus haven't had the economy to support a transmogrification market. Actually, some of the economies are virtually nonexistent when it comes to auction house. So part of my preparation is bagging up all the goodies, clearing out the mailboxes, and then waiting for the patch to hit..
I've been playing the auction house since just before transmog hit the scene. I started out trying to sell crafting materials and flipping (buying and reselling) enchanting materials that were being listed way below market. That was fun, but I really didn't have a passion for it.
After I saw all of the sets that were available while collecting my own transmogrification items from the Burning Crusade instances, I couldn't resist seeing if I could sell those very same items. As it turned out, yes I could, and I still do. I even just came across my old stock booklet I made that showed what I had, what I wanted, and what I thought would be hot sellers.
Go figure, the stuff sells, and it sells REALLY well. If you want to get into the market, now is the time to collect. After 5.4 is live, a lot of players with gold burning a hole in their pockets will finally have items to purchase with that gold and players to show their sets off to. Win for the buyers, and a big win for the sellers. :p