API Metering
Backups & Snapshots
Counters
Cloud/Cluster Management Tools
Instrumentation/Monitoring (Ganglia, Nagios)
Failover
Node addition/removal and hashing
Autoscaling for cloud resources
CSRF/XSS Protection
Data Retention/Archival
Deployment Tools
Multiple Devs, Staging, Prod
Data model upgrades
Rolling deployments
Multiple versions (selective beta)
Bucket Testing
Rollbacks
CDN Management
Distributed File Storage
Distributed Log storage, analysis
Graphing
HTTP Caching
Input/Output Filtering
Memory Caching
Non-relational Key Stores
Rate Limiting
Relational Storage
Queues
Rate Limiting
Real-time messaging (XMPP)
Search
Ranging
Geo
Sharding
Smart Caching
dirty-table management
The second section, which I call “above the line” are common application level components that typically depend on one or more of the components above. There are of course a huge list of features for any component, but I’ve highlighted some that either aren’t commonly implemented or are particularly important:

AuthX (AuthN + AuthZ)
Capabilities
Multifactor Auth
Rate Limiting
Signup
OpenID
OAuth
External import
Groups
Invites
Lists
Notifications
Spam filtering
Multi-protocol routing
Fine-grained controls/rules
Presence
Social Activity Log (Newsfeed)
Filtering
Social Model
Connectivity (uni/bidi)
Privacy (private, reciprocal, public)
Views
Traversal
Social Object
Privacy, Social Scoping
Voting
Sharing
Publishing
Comments
Favoriting
Social editing
Permissions
Tagging
Combinations
Relatedness
User
Achievements/Awards
Activity Log
External User ID Mapping
Permissions (see AuthX)
Deletion/Archival
Flagging
Direct Messaging
User Cards