Found this good article on Tech Target Network about SharePoint Online limitations and wanted to share it with you guys ….
Depending on the Office 365 plan your business uses, there are several SharePoint Online limits to keep in mind when planning a SharePoint migration.
As with most software as a service deployments, Microsoft imposes limits on the SharePoint Online service across its various Office 365 plans. Organizations with the appropriate service plan can typically work within those limitations, but it’s important for IT planners to recognize restrictions in advance and ensure that any migration from a local SharePoint deployment to SharePoint Online will remain within the established limits. Otherwise, the online migration may experience problems or incur unexpected premium costs for additional resources.
SharePoint Online limits typically involve storage and maximums in the number of items, users, subsites and file sizes. Consider SharePoint Online in Office 365 Business Premium, which offers base storage of 10 GB per tenant plus up to 500 MB per subscribed user; additional storage can be purchased for a fee. For example, a business with 5,000 users would see a storage limit of about 2.5 TB. This might sound like a lot, but storage-intensive file collections across many users can easily consume a significant portion of that capacity. Other factors like Recycle Bin storage can also count against storage limits.
The Office 365 Business Premium plan imposes other limits such as up to 1 TB per site collection or group document library, synchronizing up to 5,000 items — such as folders and files — in site libraries, synchronizing up to 20,000 items in the OneDrive for Business library, and handling up to 500,000 site collections where each site collection can support up to 2,000 subsites. Users can upload files as large as 2 GB per file, but can only attach files up to 250 MB.
There are also SharePoint Online limits on site elements such as lists and libraries, pages and security. For example, a single wiki or webpage can only contain up to 25 Web parts, and a single user can belong to up to 5,000 security groups. Such limits may require changes to existing local SharePoint resources before migration.
The limits for other plans such as SharePoint Online in Office 365 Enterprise, Education and Government may differ from the Business Essentials or Business Premium plans. It’s always important to check the needs of your SharePoint deployment against the preferred plan to ensure that you’re getting the most appropriate and cost-effective service. It’s also worth checking the site elements to see which, if any, sites or wikis may require changes.
Vignesh's SharePoint Thoughts 