I had to smack my forehead last night when I realized that my level 55 warrior was still carrying around nothing bigger than 10 slot bags. It seems so obvious that I'd have upgraded those by now, but inbetween all of the PVPing, questing, instance running, and grinding, it just plain slipped my mind. Herewith, five things that seem really obvious to remember, but a lot of players end up missing.
1. Upgrade that equipment! My forgetfulness is your gain. Bags are important to upgrade (because even if it costs you all your gold, you'll make it back quickly with the extra carrying capacity), but it helps to check all of your equipment every time you ding. A guildie of mine had to laugh when he hit level 46 on his hunter-- he realized he was still wearing a leather chestplate, despite having trained to wear mail six levels ago.
2. Train those skills. It's happened to all of us. You ding, head back to the trainer to train up, and you find that there's a ton of new skills sitting there for you. You forgot to train last time! Blizzard should have a little optional icon on the UI when there's a new skill for you to go get (or maybe this is a job for a plugin...).
3. Professions, too. Professions have even more skills and recipes to train up than the regular levels, so it seems like people forget them all the time. And for some reason, when I'm looking for herbs or mining, I always forget to turn the little tracking marker on after I die (Gatherer helps out a little with this one).
4. Repair, repair, repair! There's nothing worse than heading into an instance after LFG for half an hour, only to wipe once and have your gear turn yellow (or, even worse, red). By now, I'm repairing every single time I land from a griffon ride. Every time I enter a new town, the blacksmith is the first person I talk to.
5. LRN2PLAY. Of course, this is what all those hardcore players will tell us forgetful types, but this time, they're actually right: studying up on a class's strengths, weaknesses, skills and abilities will make things much, much easier. If you're a priest, don't use Shield-- use Renew, and less mana. If you're a Shaman, use Rockbiter (until you get Windfury, at least). If you're a Warrior, make sure to keep Battle Shout up 24/7.
Of course, all this stuff will seem obvious-- until you realize you've forgotten something. But in this game, as in everything, the devil is in the details.