I have been a bit slow getting started with my blog but here it is my first post! 
This post covers what features and other items are configurable at each level - I do not believe this list is complete. Therefore if you have more to add to this please comment on my blog and I will add it on.
| Feature |
Description |
Level |
| Data Connection Library |
Adds Data Connection Library feature |
Farm |
| Excel Services Farm feature |
Adds farm-level Excel Services Features to the Windows SharePoint Services framework |
Farm |
| Global Web Parts |
Installs additional web parts common to all types of sites. |
Farm |
| Spell Checking |
Enable Spell Checking in WSS list-item edit forms. |
Farm |
| Worker Process isolation |
Faulty code could bring down applications by web app, this is less likely at any other level. Also security breaches would be per web app. |
Web App/App pool |
| Office Server Enterprise Search |
Uses the Office Server Search Service for search over broad enterprise content. In addition to list and site scopes, provides search over people profiles, business data, remote and custom content sources. Uses multiple tabs to display results in the Search Center. |
Web App |
| Office Server Site Search |
Uses the Office Server Search Service for site and list scoped searches. |
Web App |
| OSS Enterprise web application features |
Features such as the business data catalog, forms services, and Excel Services, included in the Office SharePoint Server Enterprise License |
Web App |
| OSS Standard web application features |
Features such as user profiles and search, included in the Office SharePoint Server Standard License |
Web App |
| <Specific workflows> |
Specific workflows are made available at a site collection level. For custom and OOB workflows. |
Site Collection |
| OSS Enterprise Site collection features |
Features such as the business data catalog, forms services, and Excel Services, included in the Office SharePoint Server Enterprise License |
Site Collection |
| OSS Publishing Infrastructure |
Provides centralized libraries, content types, master pages and page layouts and enables page scheduling and other publishing functionality for a site collection. |
Site Collection |
| OSS Search web parts |
This feature uploads all web parts required for Search Center |
Site Collection |
| OSS Standard site collection features |
Features such as user profiles and search, included in the Office SharePoint Server Standard License |
Site Collection |
| Reporting |
Creates reports about information in Windows SharePoint Services. |
Site Collection |
| Quota |
Quotas for the amount of space allocated. |
Site Collection |
| Navigation |
OOB navigation is per site collection |
Site collection |
| Site directory |
List of links to sites. A site can be added to site directory at point of creation. There is a site definition for Site directory. |
Site collection |
| Use Custom search scopes |
Can manage custom search scopes available |
Site collection |
| Search keywords and best bets |
Add keywords (synonyms) and best bets. |
Site collection |
| Second level recycle bin |
Also known as Site collection recycle bin, where files go when they are deleted from Site (end user) recycle bin. |
Site collection |
| Site collection usage reports |
Usage report across the site collection. Nice graphs and tables summarising data. |
Site collection |
| Audit settings |
Change settings for what events are audited E.g. View, Edit, Delete etc. |
Site collection |
| Audit log reports |
Run reports on auditable events. |
Site collection |
| Site collection policies for retention |
Create and manage retention policies available. |
Site collection |
| Site templates gallery |
Gallery of sites saved as a template. |
Site collection |
| List templates gallery |
Gallery of lists/libraries saved as a template. |
Site collection |
| Workflows |
Which workflows are available for use |
Site collection |
| Web Parts gallery |
Gallery of web parts available for use |
Site Collection |
| Permissions |
Ability to control access to specific content |
Site Collection |
| Permissions |
Ability to control access to specific content |
Site |
| OSS Standard site features |
Features such as user profiles and search, included in the Office SharePoint Server Standard License |
Site |
| Team collaboration lists |
Provides team collaboration capabilities for a site by making standard lists, such as document libraries and issues, available. |
Site |
| Translation management library |
Create a translation management library when you want to create documents in multiple languages and manage translation tasks. Translation management libraries include a workflow to manage the translation process and provide sub-folders, file versioning, and check-in/check-out. |
Site |
| Regional settings |
Time zone, work days, time format, calendar format, etc. |
Site |
| Usage reports |
Similar to usage reports for site collection but at a lower level. |
Site |
| Appear in search results |
ASPX pages, whole site |
Site |
| Search scope for related links |
Allows site administrators to control the content of the related links search scope. |
Site |
| View user alerts |
Allows the site administrator to view who has alerts setup on the site and for what. |
Site |
| Content and structure view |
This allows users with permission to access this tree style view and move documents around (including versions) and other tasks. |
Site |
| Content and structure logs |
Audit trail of events occurring in the content and structure view. |
Site |
| RSS feeds |
Control use of RSS feeds from this site, edit copyright details etc. |
Site |
| Audiences |
Enable ability to target content at specific groups of people |
Site |
| OSS Enterprise features |
Features such as the business data catalog, forms services, and Excel Services, included in the Office SharePoint Server Enterprise License |
Site |
| OSS Publishing |
Create a Web page library as well as supporting libraries to create and publish pages based on page layouts. |
Site |