What phones are you targeting? Any phone released in the last 3-5 years is able to view standard (X)HTML without issue - except for the fact that they mostly have very small screens.
Some phones do not support Javascript very well (but that is mostly changing, too) and most phones do not support Flash (yet - most newish phones except iPhone will probably support flash "soon", if not already).
The only other main difference is that most phones these days use a touch interface, rather than a traditional keyboard/mouse. That means you can't really do things like hover effects and so on.
But other than the smaller size, everything else is pretty minor.