There have been several infamous patents for software techniques which most experienced programmers would consider fundamental or trivial, such as the idea of using exclusive-or to plot a cursor on a bitmap display. The spread of software patents could stifle innovation and make programming much harder because programmers would have to worry about patents when designing or choosing algorithms.
There are over ten thousand software patents in the US, and several thousand more are issued each year. Each one may be owned by, or could be bought by, a grasping company whose lawyers carefully plan to attack people at their most vulnerable moments. Of course, they couch the threat as a "reasonable offer" to save you miserable years in court. "Divide and conquer" is the watchword: pursue one group at a time, while advising the rest of us to relax because we are in no danger today.
Compuserve developed the GIF format for graphical images many years ago, not knowing about Unisys's 1985 patent covering the LZW data compression algorithm used in GIF. GIF was subsequently adopted widely on the Internet. In 1994 Unisys threatened to sue Compuserve, forcing them to impose a sublicensing agreement for GIF on their users. Compuserve users can accept this agreement now, or face Unisys later on their own. The rest of us don't have a choice -- we get to face Unisys when they decide it's our turn. So much trouble from just one software patent.