I've not seen the stuff on Lynda - I find most value out of searching YouTube for 'drupalcamp' and 'drupalcon' as most of the sessions are on there so if there's something specific I need I can usually find a recent session on it (be careful on 8 as much changed so make sure you have the latest).
The big change from 7 to 8, and the reason it took so long, is going from procedural to object orientation so they're quite different. In 7 there were many 'best practices' whereas in 8 there's a pretty much standard way of doing things which is more in line with modern day programming techniques - it's not totally there yet but a vast improvement on 7.
For front-end stuff on 7 the issue is working out where stuff is being generated from - if you think of the hook system like a coat hook there's often many coats on a hook so you change something, wonder why it's not changing on the front-end, clear the cache, it still doesn't change, then you find someone's done some business logic in the them layer, etc. etc. whereas with 8 there's restrictions on what you can put in the theme. That's as far as I'll talk about front-end as I'm very much not into CSS, I do logic programming and to me the whole browser wars issues drives me nuts, I like my business logic to work across any device, so stick to back-end development, APIs, etc. For front-end be sure to watch MortenDK's videos, he spent many years working hard to change the face of Drupal theming as before it was very much created by devs for devs.
With regards to architecture differences between WordPress, I think it's unfortunate they get lumped into the same 'CMS' category - WordPress started off as a blogging platform and due to their wish to provide eternal backwards compatibility they're kinda stuck with it, whereas Drupal dares to change every major version in order to make the most of innovation, so to me it's light years ahead. Drupal is a web app development framework which happens to have content management capabilities, it also can do a lot of other stuff too and the real power is the integration of these, whereas WordPress, along with its different ecosystem of paid modules as opposed to modular and free (although I realise many WP modules are free), you often end up with what someone called 'Frankenblogs'! Distributions are also key to Drupal, providing specific use-cases like
https://www.getopensocial.com - however these distributions are often harder to extend so better where your use-case is mostly filled by the distribution than as a starting point. That said, there are ones like
http://lightning.acquia.com/ and
https://www.drupal.org/project/thunder which are more geared to a starting point for development.
One thing that I had to get used to - and still am not really, is that a lot of the discussion goes on IRC (
https://www.drupal.org/irc) - many UK contractors and freelancers hang around on #drupal-uk, so that's good to ask questions on when people are around, and there's specific ones for commerce, rules, etc. But by far the best is getting round tables IRL & working on stuff - I've been to many DrupalCamps and DrupalCons around the world and am always amazed at how awesome everyone is, and often you can get answers to your burning questions in seconds. There's also stackexchange which has a drupal channel - sadly the forums on drupal.org aren't officially supported any more however a few people are doing their best to answer questions posted.
Givers gain, I've certainly learned a lot through helping out where I can - I still don't do as much as I'd like and am in awe of the amount of hours and effort people in the community put in to keep the largest open source project in terms of contributors going. Drupal enables anyone to make use of human's best invention the internet - I met an ex-prisoner at DrupalCon Chicago who was using the OpenAtrium distribution to help other ex-prisoners on their 12-step programme & saw DrupalCon was on so came down to see what else it could do. I believe having a platform like this will revolutionise the web - of course not everything has to be done in Drupal but not everyone who wants to use the web needs a customised F1 racing car of a website, many just want to get from A to B using a Ford Focus, etc. and that's where Drupal comes in.
I'll stop ranting now... enjoy your travels & be sure to participate in the community & you'll get a lot more out of it - one quote I remember is 'Drupal is the best community to have a problem in'!