You should also consider the compatibility of your browser's version with Salesforce. When browsers do an update to their version, some Salesforce features may not be compatible to the version yet. It usually takes few weeks or months before the compatibility is fixed.
As of today, here are the supported browsers:
a. Internet Explorer - versions 7,8,9, and 10.
b. Mozilla Firefox - most recent stable version.
c. Google Chrome - most recent stable version
d. Apple Safari - versions 5.x and 6.x on Mac OS X
3. Salesforce Edition
60-80% of the issues being received by Salesforce Support are Edition-related limitations. If it seems that you're missing some features and functionality, or if your 3rd party applications from the Appexchange fail to install, then you might want to check first if they are supported with your edition.
Here are the common issues that are reported and found to be Edition-related:
a. Custom Profile - Cloning or Customizing Profiles are not available for Professional Editions and below.
b. Mass Email - This feature is only available for Professional Editions and above.
c. Permission Sets/ Sharing Rule - Although Account and Contact Sharing Rules are available in Professional editions, sharing rule to other objects are only available for Enterprise Editions and above. Consequently, Permission Sets are only available for Enterprise editions and above.
d. Workflow and Approvals - Although this feature may also be available for Professional Editions with additional cost, it is only available by default for Enterprise editions and above.
e. API - Some applications might require you to have an API enabled. API is only enabled by default for Enterprise editions and above.
To get the complete list of features and limitations, please check this documentation: https://help.salesforce.com/HTViewHelpDoc?id=limits_general.htm.
4. Unable to Receive Emails from Salesforce
If you aren't receiving important emails from Salesforce, like Password Rest emails, you should initially check if your network setting is blocking any IP addresses coming from Salesforce. To determine: Name | Setup | Email Administration | Test Deliverability | Enter your Email Address | Click Send. You should receive a total of 52 emails. If you receive less than that, then you confirm that you are blocking IPs from Salesforce.
To check the IP addresses that need to be whitelisted, please refer to this site: https://help.salesforce.com/HTViewSolution?id=000003652&language=en_US.
5. Setup Audit Trail
Salesforce Support is also receiving lots of cases regarding the questionable changes in the customization of their organization. These changes are not always Salesforce-related, most of the time Support would just figure out that one of the company's System Administrators were the one who did the change. Setup Audit Trail could help you in determining the configuration changes that had happened in the past 180 days. To run an audit: Name | Setup | Security Controls | View Setup Audit Trail.
6. Recycle Bin
For missing records, you may want to check if they were just accidentally deleted by one of your users. Your recycle bin stores deleted records for 15 days.
7. Blogs, Community Pages, and Help and Training Portal
You could also consider finding your answers first from one of Salesforce experts in different blogs in the internet. Most of your answers could also be found on the Help and Training portal. These resources are readily available and should be maximized to find answers to your questions.
If all of these recommendations fail, then you might want to consider logging a Support Case. In order to resolve issues that could potentially impact your business, you should be familiarized on the first steps that should be taken into consideration. As you become expert in handling these common issues, soon you will turn into a walking Salesforce encyclopedia!