Tech isn't the hard part - barriers to get a service up and running are dropping drastically.
Hard part is marketing & scaling, and as AWS expands services there are risks from pure-play vendors that do just one slice of the stack 2-3x better than Amazon does.
1. OpenStack still has plenty of bugs and issues you are going to hit.
2. You'll still need a really good colo facility.
3. High bandwidth and Multiple internet providers are expensive. If you look at Amazon they have an excellent blend of bandwidth including direct connectivity to many eyeball providers (Comcast, Charter, RCN etc.) which usually makes your data get there quickly and prevents peering bottlenecks.
4. If you need more than one facility then you just get to double your costs.
5. People costs, you'll need people that can manage your infrastructure at scale.
6. Hardware capex can severely drain companies.
7. Vendor sprawl and support. (You still may need a CDN, DNS provider, etc.)
Doing your own thing can quickly become much more expensive than Amazon or other cloud providers.