1) How important is having a 'big name' employer on your resume? (I feel what you do is more important than who you work for, do B-Schools see it that way too?)
A big name helps because program's will have familiarity with blue chip firms and their hiring process is almost like a vetting process for top MBAs as well. However, the impact you have made at your firm and the results you have achieved is more important so keep that in mind.
2) How important is the amount of work experience you have? (I've heard the average is four, but I'm sure it varies from one school to another).
Programs want to make sure you have had meaningful work experience and developed during your time working pre-MBA. Programs want to ensure you can add something to classroom discussions and the overall student community. If you can convince them of this in your application then generally anything +2yrs is in play.
3) Undergraduate college: Does it matter that I went to a non-prestigious 4-year college?
Look at it as more holistically its about a combination of things - school caliber, GPA, major, quant class performance. The importance of these factors slide up and down depending on how strong of these are. But of course a more prestigious school will give you more leeway because a 4.0 from Harvard will be more impressive than from a Trucking School
4) Extracurriculars: How important is it to have community service on your resume?
Important, shows you are well rounded and provides unique insight into your interests and personality. Also this tends to be a great way to highlight your interpersonal skills, especially for younger candidates
5) Does my industry matter? Maybe I'm just overthinking it but I feel like accounting is seen in a more poor light than economics or engineering.
Not really, it's about the impact and your results. Showcasing an analytical aptitude from your role helps here