Customizations Possible (Dev) in a SharePoint 2010 Environment

WebParts
That use only DLLs
That use .ascx files (deployed in ControlTemplates folder)	Web.config GAC 12 hive (only when webparts are deployed as .ascx files)
Workflows
SPD Workflows	Will automatically get migrated, because it is there in the content db Fur between SP 2010 Farms, if you need to migrate, you need to use tools – Workflow Migrators
VS Workflows	Can be just solutions – can be just redeployed GAC
Custom Master Pages	Content DB is fine unless, the application default.master or v4.master is modified (which is in layouts folder)
Application Pages (Custom Layout Pages)	Layouts folder Straight copy
Features	Features folder (Manifest.xml and Elements.xml)
Solutions	Can comprise multiple features Can be found in Solution Management of the Central Administration
Third party add-ons	Re-install (depends on what kind of add-on that is)
Custom Security Trimmers	GAC – assembly

Few other things:

Event Receivers	Could be deployed as Features / Solutions
Custom Site Definitions	Under SiteTemplates
Custom List Definitions	Not any folder, but GAC again
Custom Content Type Definitions	GAC
Custom Images	Images folder
Custom Icons and File Types	Central Admin, Search – File Types DocIcon.xml under Template/Xml folder
Custom Fields (Custom Columns)	FldTypes.xml in the Template /XML Folder
InfoPath Form Templates	Locally stored in each development (.xsn files) – this is not a good practice, it might be directly present in the Content DB
External Data Connections	Worry about the connection string ODBC Connections – you will have to re-create them in the SP Server
Data Connection Files stored inside Libraries (pointing to a wrong location after upgrade)	Content DB