2. The characters are usually caricatures. This is probably in part because finding people who can both sing and act is very hard. It doesn't explain why a huge percentage of musicals involve some sort of ethnic cliché or stereotype.
3. The action stops so the lead can show off his voice. I don't mind musicals in which the songs are funny (Avenue Q, South Park), move the plot along (some songs in Moulin Rouge!), or reveal new insights about the character. Most of the time, though, the songs just reinforce what we already know. For example: what the point of "
4. You can't understand what they're saying anyway. If a song does contribute to the narrative or characterization in a meaningful way, it's uncertain whether that will come through, since the singers often are too wrapped up in showing off (good voices) or are unable to combine enunciation and volume (bad voices).
5. Songs are often jammed into a perfectly good story. This happens a lot with musical adaptations. It throws off the pacing.
6. The sort of person who loves musicals makes me want to die. You know who I'm talking about. That girl with the Les Misérables t-shirt in 9th grade. That guy in college who looked down on people who didn't appreciate Zero Mostel's hilarious songs. The person who always manages to drop the name and price of the show s/he saw in NYC last weekend. If you watch musicals you're around these people. Ugh.