I'm annoyed that there is no simple system for the IRS to tell me what I owe, and let me pay or dispute. Instead, we play this game where the IRS provides a bunch of complicated documents, I guess which ones to fill out and submit, then wait 3 years to see if I was randomly chosen to see if I was correct. So it's very easy to make a mistake for dozens of years, then suddenly be penalized at random. It's the worst kin…
Austrian tax returns are much simpler because in the majority of cases they don't exist: Your employer and your banks automatically withhold the correct amount of taxes (capital gains are taxed at a flat rate and don't influence regular income taxes). Marriage status doesn't matter for income tax as the same tax brackets apply to everyone. Deductions are much less common but if you have any deductions you can let the government know and they'll automatically refund your tax within a week or so.
One reason why Austrian taxes are so much simpler is because there are much fewer special cases: The US has all those different credits and deductions that lower the taxes in certain situations, e.g., the "First-time homebuyer credit repayment". Meanwhile in Austria all those subsidies for, e.g., lower income people aren't included in the tax system, but are available via completely separate programs (e.g., if your income is very low the government pays parts of your rent and helps you with heating costs in the winter, but you have to submit a separate application for that).
Not sure which system is better. One thing I noticed is that in Austria I never applied for any subsidies even while being a low-income student (why would I apply for social services if I don't really need them?). Meanwhile in the US my tax advisor makes sure that I get all the credits and deductions that I'm eligible for.