The plugins store your workspace configuration. If you delete them then your workspace may become unusable.
Depending on your plugins used and what you are doing you can probably delete some of them. Personally I'm regularly cleaning some of the plugins.
What I delete:
- maven repository files/indexes
- project indexes
- Copies of deployed files to application servers.
I'd suggest you get a tool like WinDirStat and check, which plugins use up your space and maybe by name e.g.
index you figure out that this plugin can be safely deleted.