Drupal 7 is getting mature as the time of this writting and many of its contribution modules is on stable state.
Here is a list of top 20 Drupal 7 modules that you must install to enhances your Drupal Site
 
1. Token
This module will provide you with tokenized feature. Most important for automatic pathauto pattern.
 
2. Pathauto
Without this Drupal can still provide clean urls but without any automatic pattern generation
 
3. CKeditor
You'll need this (or other similar wysiwyg type module) if you want wysiwyg editor in Drupal
 
4. Metatag
Do you need metatag for your site?
 
5. Field permissions
Somehow Drupal core developer forgot to move CCK permission per field into core, so this module will provide the same features for Drupal 7.
 
6. Media
If you want drag and drop image like wordpress, this is the de-facto module for it.
 
7. Transliteration
You'll need this one to keep sane filename for uploaded files.
 
8. Mollom
The most effective spam killer, don't build site without it.
 
9. Backup & migrate
Enjoy simple point and click backup and upgrade. No need for PhpMyAdmin.
 
10. Boost
Shave your site loading time by half (or more in some cases) by caching Drupal into a static html file. Some advanced user may perfer varnish + memcached but IMO Boost is dead simple and effective. The best of all is it will work on shared hosting.
 
11. Display Suite
Stop doing tpls! utilize DS to format your nodes layout.
 
12. Filefield path
Allow you to input a path for file instead of uploading it
 
13. Filefield sources
Allow you to use other sources as file source instead of uploading them
 
14. Admin Menu
IMO the most usable Drupal menu for administration.
 
15. Devel
Don't build site without it
 
16. Taxonomy Display
Enhance your taxonomy display with this module.
 
17. User Dashboard
Lets non-admin user enjoy the same dashboard in their profile page
 
18. Views
Need to quickly build custom blocks?
 
19. Webform
Build custom contact form easily.
 
20. Field Group
Group your field into tabs.
 
Sure the list can go on and on.... but those are top 20 modules that you should have for your site.