I think most of us who are opposed to this tax bill which I call the "Make Trump Richer Bill" oppose it because:
1. It is lopsided with only 16% going towards personal income tax cuts. Around 12% benefits people who inherit estates in excess of $11 million and the rest is for corporations. This will only exacerbate the income and wealth inequality in this country.
2. The personal income tax cuts decline over time for lower income people. Poor and middle class people who receive a tax cut in 2018 see these tax cuts shrink over time and many end up paying more in taxes including the middle class family Paul Ryan used to promote the bill. Mr. Ryan claimed a family of four earning $59K would get a $1200 tax cut. In fact, they only get this the first year and by year 10 they are paying an extra $500 in taxes..
3. The bulk of the personal income tax cuts are enjoyed by the richest 5% and especially the richest 1%. People earning over $500K a year make the biggest gains.
4. The tax cut costs $1.5 trillion and will not pay for themselves with economic growth. History shows that while tax cuts do result in economic growth, the growth is too modest to garner enough revenue to pay for the tax cuts. The Bush tax cuts increased economic growth enough to replace 10% of the revenue lost from the tax cuts. The Republicans are relying on future growth rates of 6 or 7% to avoid running up the deficit.
There is a valid concern that growing deficits will result in the Republicans having political cover to do what they have been wanting to do for a long time: cut Medicare, Social Security, Medicaid and make deeper cuts in non defense discretionary programs (research, environmental protection, consumer protection, State Department etc).
5. To limit the tax cut to $1.5 trillion, they have raised taxes on several groups of people most notably upper middle class (as opposed to really rich) people especially UMC families who live in blue states. This is a politicization of our tax code. It is also strange to tax families earning between $100,000 - $300,000 in order to give more money to people who earn more than them (again remember people earning less than $100000 don't get very generous tax cuts and those shrink over time whereas the tax cuts for the very wealthy stay in place).
6. They are doing away with some deductions that many families rely on including the adoption credit and the medical expenses deduction. The medical expenses deduction is used by nearly 9 million families who spend more than 10% of their income on medical expenses and it helps families who are dealing with an expensive health issue such as cancer which could have them facing bankruptcy or a long term health care commitment such as home health or nursing home care for a spouse or parent. Nursing homes in the DCUM area cost $150,000 a year and the tax deduction helps families to use their own savings to keep their loved ones in care facilities without having to go onto Medicaid.
The WP covered the story of a family in just such a situation,
https://www.washingtonpost.com/business/economy/they-spent-years-planning-to-live-with-alzheimers-disease-the-gop-tax-bill-threatens-those-plans/2017/11/04/83ac1ffa-c098-11e7-8444-a0d4f04b89eb_story.html?hpid=hp_regional-hp-cards_rhp-card-business-technology%3Ahomepage%2Fcard&utm_term=.c7f336f7885d