The DDI Adventure Tools make it very easy to refluff or restat a monster in no time. Creating a monster is still easy, but can be difficult due to trying to not create an overpowered monster. Aside from looking at suitable monsters of the same role/level, what other cautions should be taken or rules should be followed in custom monster creation?
1. Make sure you're using MM3 monster stats.
From Monster Manual 3 onward, monster stats are calculated differently. Older monsters that you modify may still have the old stats. "Soldier" type monsters will have too high an attack bonus, for example.
2. Be careful when using stun and daze effects.
Stun and daze are very powerful and frustrating. They rob the players of their actions, which is annoying and makes combat take longer. Use these rarely or never.
3. Benchmark your creation against similar monsters.
Compare your monster to others of the same level and role. They should be roughly equal overall. It needn't be an exact match, but if your attacks are significantly more powerful than an equivalent creature, you'll know it needs adjusting. Be careful when you give it more Encounter powers than normal, since a monster who can afford to use Encounter powers all combat long is more powerful than one who has to use the weaker at-will powers.
4. Make sure solos have some way to deal with stun and daze effects.
RPG blogger Mike Shea recommends giving solos a way to shrug off status effects, such as taking 25 damage to remove stun. This lets stun powers remain effective without allowing a dragon to be shut down completely for several turns. Also make sure your solo isn't powerless against certain PC types, like ranged attackers or PCs who can resist a certain element.
5. Don't make the monster too complicated.
Most monsters will only last a few rounds. Don't waste effort giving it a lot of abilities it won't get to use. This only slows down combat as you take longer to choose which power to use. It also makes it harder for the players to learn what attacks the enemy has, and players like working out what to expect.
6. Have someone else check your monsters.
Two heads are better than one. There may be a dangerous combination of powers that you haven't spotted.
7. Resist the urge to build monsters specifically to trump your players.
It's tempting to build a monster that is immune to the player characters' abilities or strikes their weak points, but the players will hate it.
8. Before you build a monster, look for another you can re-skin.
You can save yourself a lot of effort by simply re-skinning an existing monster and maybe changing a power or two. There are enough monsters in the Compendium now that you're very likely to find something suitable, even if it has to be shifted up or down a few levels. There's no way the PCs have memorized the stats of every monster in advance, and if you change the description they'll have a hard time even finding which creature you used afterward.
The biggest problem spot I saw was that some abilities don't scale according to the formula the DMG gives you. Sneak Attack, for instance, gains extra damage dice at level 11. If you take a level 12 monster with sneak attack, and scale him back to level 10 he'll still have paragon tier sneak attack damage. If you instead level him from 10 to 12, his sneak attack will be underpowered. Just make sure that abilities that scale like this get adjusted correctly, even if Adventure Tools doesn't do it for you. Usually this will happen at levels ending in 1 and 6.