In case you weren't already aware, our nation's tax code is a mess. And not your run-of-the-mill mess that can be tidied up before company comes over; this is more of a dirty-dishes-on-the-floor-ring-around-the-entire-bathroom-kitchen-smells-like-the-dumpster-what-the-hell-is-that?-is-that-dog-piss? mess. Both political parties believe that reform is needed, but that's where the agreement ends.
The Republicans would like to cut tax rates and eliminate deductions in order to "broaden the base" so that "everyone pays something." Democrats…well, Democrats aren't really known for coming up with ideas (Republicans aren't exactly great at this either) but they do a damn fine job of opposing GOP proposals (again, Republicans are also good at this; e.g. healthcare reform).
The Hill reports that one key thing that the Dems oppose is the "elimination of many tax breaks that middle-class Americans enjoy," citing the mortgage interest deduction as an example. They also aren't crazy about the Mitt Romneys of the world getting their taxes cut because rich people having more stuff = bad. One other thing that is grinding their gears has nothing to do with specific policies but rather the possibility that the GOP would hog all the tax law re-writing bliss for themselves:
Democrats also said the bill would allow the Ways and Means Committee chairman to write the entire tax reform bill by himself, which they said goes against the GOP's stated goal of opening up the process of tax reform to the full House.
Opposing tax cuts and certain deductions is expected, but whining that W&M Chair Dave Camp wants to tackle this all by himself is dumb. Let him do it. You can still vote against it it committee. No one wants to hear your horrendous ideas on tax reform anyway.
Never mind that many members of the House are barely lucid let alone capable of developing tax policy that make sense. Of the 435 members of the House, there might be a handful that are actually capable of having an adult (relative term as it relates to Congress) conversation about this issue. The Democrats wanting the debate opened to the "full House" would result in less being accomplished we currently expect on tax reform. And that's saying a lot. God, Congress is full of idiots.