Extremely simple
Beginning with Drupal is trivial. Installation is perfect in it's simplicity, there's really no way to mess up the installation :) In it's basic installation Drupal offers so much. For the beginning it offers management of everything an intermediate developer would need. With basic knowledge of web development you'll be able to put together a very powerful web. You can extend your web with thousands of themes and modules, without any manual intervention whatsoever.
Extremely deep
When you want even more you can get it all with Drupal. You can manage any data you can think of, any menus, any blocks, any categories, tree structures, files, images, galleries, feeds, forms, interactive blocks, articles, comments, anything ... For most of your needs you will probably find a module just right for you :)
Unbelievable power
There are many ways of getting so much from Drupal. You can build CMS systems on it. You can build social networks, huge intranet solutions, portals, blogs, personal webs, portfolios. You can choose any level and any size for everything. Drupal hardly has competition in this. I know there are wonderful platforms offering great features for every project type. But with the right approach you can dominate any project size with Drupal :)
Modules
Thousands of great modules available from the members of the huge Drupal community. Each module goes through really rough approval process, so there's absolutely no way of getting broken module with Drupal (if you follow common sense). You can get modules for anything, as I will explain further Drupal is extensible in every way, his genius hooks system allow to modify Drupal behavior in almost any way. This is why modules can offer incredible features making Drupal even more powerful.
Themes, blocks
In my developer career I created 3 CMS systems, I worked with several third party solutions as well, but none of them could even compare to the comfort drupal offers in theming and personalizing the user interface. You can create themes very easily and if you do it correctly you can use your themes in as many projects as you wish without struggling with the project settings. For blocks, you can display any data in as many ways as you wish, you can have separate templates for any view and this brings incredible order to every project. Everything is nicely cached, performance is wonderful and you know where to look for everything, because it just makes so much sence :)
Incredible independence of layers
I'm an OOP developer for years now and this was for me the biggest issue when I was moving onto Drupal. I thought you can never separate layers as good as you can with OOP. I was wrong. Even though Drupal is not build on OOP the separation and independence is 100%. There is some initial struggle before you learn just how to do everything correctly, but when you do, it all works together and in time you learn to build modules that can be used in another project without changing a single line of code :)
Always something new
I'm still in the process of learning Drupal, because it is so deep. I learn to improve my ways of using it practically on daily basis and I enjoy this progress. Sometimes I get to a point when I think for myself that Drupal developers didn't think of this or that, but later on I stumble upon the fact, that it is covered indedd, and it's covered down to the last detail. For example error reporting with watchdog, it's a small thing, but it's great and very easy to use. Messages, forms, blocks, rights, variables and cache that are so trivial to use and offer so much ... :D
Community
There are thousands of developers around the Drupal ecosystem. Finding answers, modules, support, themes, anything - it's not a problem. Also I can say that the community is very friendly, opened and helpful. Thank you Drupal developers for this :)
Speed, optimisation options
Drupal offers incredible tools for effective and blazing fast webs. Caching system, image cache modules, CCK, forms and templates caching and preprocessing. You can build really huge solutions with Drupal and still provide swift and fresh response for everything. This is probably the thing I admire the most about Drupal, since knowing (and still learning) how many features and options it offers I have to say that it's speed is fantastic :)
Mindblowing flexibility
Ok, last one :D Well, as I mentioned before, Drupal offers great hooks system so that with each module or theme you can alter Drupal behavior. You can affect pretty much everything you could need from within your module. Other than drupal I don't know any CMS offering such flexibility. This is what makes the reusing so wonderful, because when you take one module that improves your web on 10 fronts and install it on another web guess what. It's instantly as powerful as the old one.
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Well, that's about it, I am well aware that this is very positive testimonial and that I focused only on the positive. But, I love Drupal and if you don't that's your bad :) I know, for years I have settled for one platform, then another and said, this is the best ... And there was always something better ;) But, you know passion, enthusiasm ... For now I couldn't imagine something better for me to develop anything than Drupal :) Peace my friends :D