Here are examples of things I consider simple:
- W2 income and you're <= 24% income tax bracket
- most of your investments are in tax deferred/tax advantaged accounts (401k, IRA, etc)
- mortgage, car loan, etc
- married filing jointly, head of household, or single
- modest amount of charitable donations
And here are some things I consider "complex":
- married filing separately because you think it's more efficient for you
- owner of a business
- multiple real estate investment properties
- lots of after tax investments (5 figure profits on an average year)
- 1099 income with lots of expenses
- regular gambling income or other "irregular" income
The vast majority of people have "simple" taxes, so you really need a good reason to think you need a professional. That said, paying once and having them walk you through it can be educational.