I am not denying that there may be some need for some primal "evil" and "good" forces in our fantasy genre. In some cases, an "evil" demon may make a good plot device. However, such evil could be described in other ways--no regard for life, likes to cause others pain, selfish, etc.
Now I'll list my reasons for not loving the alignment systems in D&D.
1. Confusing: Confusing for new players. Alignment is not an intuitive model for describing character, personality, motives, or loyalty.
2. Arguments: Creates unwanted opportunities for arguments/debates, especially out-of-character arguments. This halts actual play.
3. Low utility: Not especially helpful to fleshing out a rich tapestry of characters, setting, or story.
4. Game exploits: Game exploits turn alignment into just another min/max advantage via detect evil, protection from evil, smite evil, kill evil, and justification for doing so. As others on this thread have said, these kinds of spells can make many complex plots/problems, that would otherwise be interesting to play out, boil down to detect evil--kill evil.
5. Alternatives: There are many other ways to flesh out characters that are more compelling. Character history, personality traits, phobias/fears, likes/dislikes, loyalties, allegiances, greed, etc.
6. Reinforces racism: Assigning an alignment normative range (or behavioral norms) to a particular race in our fantasy genre reinforces racism in the real world. V a n D y k e wrote an excellent blog that touches on this point here: raceindnd.wordpress.com/ (look for his Nov. 18 piece half-way down the page)
7. Evil: "Evil" is a hot-button button issue for D&D "haters". Sure, other things are too.