Although Salesforce have opportunities objects and opportunity line items, there are use cases where it is tempting or even necessary to go down a different way and model the opportunity through a custom object, as discussed for example here:
https://developer.salesforce.com/forums/?id=906F00000008jzgIAA
What are the features you give up when going this way? Are there significant drawbacks in adopting this approach?
What you're specifically giving up depends on which features you have enabled in your org. A non-exhaustive list of potential features, off the top of my head: Territory Management, Dated Currency Rates, Customizable Forecasting, Sales Teams, Competitors, Partners, Contact Roles, some types of Sharing, Stage History, some types of reporting, Campaigns (e.g. ROI and Campaign Influence), Price Books/Pricing, Lead Conversions, Quotes, Quotas, Products, and a number of quality-of-life features you'd have to build if you want them. Some of these features you can replicate in code, some you cannot. For all the "core" objects (Account, Contact, Lead, Campaign, Opportunity, etc), I'd strongly advise trying to purpose them to meet your needs. Going custom is likely going to cost you in the long run, so I would consider the situation carefully before doing so. Note that I'm not saying "never, ever do this", just a strong word of caution; it would be easier to try and make it work than go with a custom object.
