Minecraft wiki entry for Improving_frame_rate, associated text: Try not to go to the Nether on lower-end PCs. Particularly complex terrain like mountains or jungles can also be troublesome.
If in multiplayer, move away from areas densely populated by players.
Large numbers of item frames in view can cause lag. This includes map walls!
Be careful about building large farms or other devices.
While crop growth is not usually burdensome, for sufficiently large farms even plant growth can add up.
Farms that produce large amounts of animals or item entities can cause lag.
Large and complex redstone devices, especially those driven by fast clocks, can likewise make trouble. Keeping redstone dust and other changeable blocks out of sight can be helpful.
Rapid changes in lighting are especially troublesome. Several redstone devices emit changeable light: Even switching a redstone torch can require updating the light levels for blocks up to 6 spaces away. Redstone lamps (and mobile glowstone or other lights), can affect much larger areas, especially if they are illuminating a complex landscape or build.