Cross-posted from my WotC forum:
I think of everything in the game, solos have the most potential for a frustrating fight. It's important that a solo fight take place in an environment that encourages movement and interesting choices. Otherwise, you end up with the party standing around, trading blows with one monster. Ideally, solos are designed themselves to encourage movement, but that isn't always possible or plausible.
For example, let's take the dragon fight I designed for the Kobold Hall adventure. Were I to do it again, I'd add the following to make the fight more interesting:
* An icy ledge around the edge of the chamber that a character can climb up to snipe at the dragon, but the dragon can smash the ice and send the PC tumbling to the floor if he doesn't keep moving.
* Pillars of ice that provide cover, but that can be destroyed with attacks to cause minor cave-ins.
* Pools of water that flash freeze when the dragon breathes on them, trapping PCs for a round. However, the dragon tends to stand so that the pools are the best places for flanks, and the dragon can also be forzen in place with cold attacks.
* A visible treasure on the opposite side of the room, partially frozen in the ice, that includes a weapon or implement that would be useful against the dragon.
* A thick snow drift that a PC can dive into, hide within, and then spring out of to attack the dragon.
* Icicles hanging from the ceiling that a character can knock down to impale the dragon.
I think that when you have a solo fight, you should look for terrain and other features that:
1. Encourages movement.
2. Rewards risk taking (standing here might be a bad idea, but it gives a real advantage for at least a short time).
3. Gives benefits for doing something other than attacking (spend a round to get that nifty +2 sword).
4. Gives the characters "virtual powers", set pieces that you can use to attack so that the characters aren't just grinding through the same sequence. You can also create these to do some damage on a miss, to doubly reward PCs (they had a good idea, and even on a miss that pays off).
5. Find uses for powers that the solo might shut down, like the freezing water example above. Your ray of frost might not hurt the dragon, but you can still freeze it in place.
In many ways, think of your terrain as filling all the gaps, and roadblocking any potential lame-ness, that might come up in play.