1: It’s 4 products not 3
Many people look at Office 365 and think its 3 products; Exchange Online, Lync Online and SharePoint Online and approach it as 3 products. Its better to look at it as 4 with the 4th being the identity process also known as Windows Azure Active Directory.
It is important that you approach these as 4 different products with Azure AD as the first of these. If you ensure your Azure AD and if you implement it, DirSync and ADFS are perfectly configured, you will have far less issues in Exchange Online and SharePoint Online. You can also sign up to Azure AD without Office 365 if you want to get that process started now.
2. ADFS and the sts.domain.com
Who said that your ADFS authentication website has to be sts.contoso.com? You only follow the instructions from Microsoft and TechNet and they show it has to be sts.domain.com in their example. Why not have something a little more user friendly like login.contoso.com or cloudlogin.contoso.com? Do you know what sts stands for? 
3. You data is about to be deleted!
Well its not and the email is confusing. When you sign up for a 30 day trial you are given 25 E3/A3 licenses but you may want a different plan such as A1, E2 or A4. At the end of the 30 days you will purchase licenses and if you don’t purchase the 25 trail licenses they will be removed. The email says your trial data will be deleted. The data is only the licenses nothing to do with SharePoint or Exchange content. Just make sure you have purchased licenses and no one is using the 25 trail licenses.
4. Being clever with licenses
Lets say you have 100 staff and one of your requirements is Visio Services in SharePoint Online. Do some research in your organisation to find out if everyone needs access to Visio? Having done your research you find that only 10 people need Visio while the others just need standard SharePoint.
When your SharePoint Online is created, you are given all the tools to develop the enterprise features such as Visio Services and they don’t go away if you only go for A2/E2. The enterprise features will only work for those who are licensed. If only 10 of the 100 need enterprise features, only buy the licenses you need and assign them to the right people.
5. Office Pro Plus Licenses
If you are buying A3/E3 licenses because you get Office Pro Plus check the price difference between the yearly cost in O365 to the annual cost of your Microsoft agreement. You may find that it is cheaper to buy them through your school agreement/Microsoft partner rather than through Office 365. For those who are education, you are most likely going to find it is cheaper through an EES agreement rather than purchasing E3 licenses in Office 365.
6. Office 365 doesn’t mean SharePoint Online
As pointed out in number 1, Office 365 is 4 products. If you just want Azure AD, just get Azure AD, you don’t need Office 365. If you have pressure to move to Office 365 from your CTO, just do Exchange Online, or just Exchange and Lync. Going with Office 365 doesn’t mean you have to implement all of the service and then pay for them. Only need Exchange, only buy Exchange Online licenses.
7. Internet Connectivity
You need to ensure you have a robust internet connectivity and if it’s that important, a decent back up as well. Moving to the cloud mean you are going to use internet service more. Can your internet cope with additional traffic outbound and inbound? What if the water engineer outside accidently cuts the fibre internet into your building. How long can you business cope without Email, Lync and SharePoint?
One of the responses I heard to this was, we’ll send them all home and they can work from there and just use their own internet connection, that’s why they have laptops. It wasn’t until I pointed out that their ADFS server used the same internet line to authenticate all their users from outside of the network they decided something had to change
8. Upgrades – THING CHANGE!
Having managed a few tenancies through the process of the 2010 to 2013 tenancy upgrade process, Microsoft will do changes and they will let you know when these are through the planned maintenance through the Office 365 Admin portal. But be careful, Microsoft might change something without letting you know. Be prepared just in case you need to send out a quick SharePoint Online fix. If you want to know my experience, give me a call and I’ll happily tell you the stories.
9. Email Notifications from SharePoint
They all come from the same email address. It doesn’t matter what farm you are on, your location in the world or data centre, it will come from no-reply@sharepointonline.com. You can’t change it
10. External Contact and accessing content throughout your site
The external contact feature is great to invite individuals to share documents and sites but what if your site has content that is cross referenced to other sub-sites? You need to make sure that when you are sharing content to an external contact, they also have access to anything else that links from that site (if you want to see it). If you have heavily customised the look and feel of SharePoint and the external contact doesn’t have access to the company logo stored in the root site collection, it won’t show.