[Edited]
Mark,
I am afraid the differences are substantial. However a lot of functions we needed plugins in TWC days for are built into TW5. So once you overcome some conceptual hurdles TW5 is easier and much, much more powerful.
Personally I chose to develop my TW5 skills separately and continued using my classic wikis (using tiddlyDesktop), I then migrated some of my classic tools to tw5 then bit the bullet and built a whole replacement for my classic wikis.
It has being a long journey but I hope those of us who have already can help smooth the way for you.
There are tools for import of TWC content but I personal did not go down that path except for some high level stuff. I "declared a new epoch".
Whilst in the beginning "empty line breaks" was a conceptual or emotional barrier that is all but forgotten by me now. If I have a block of typed text I wrap it in triple Quotes or put it in a text tiddler (Rare but I can). There is also code format blocks and more.
"""
Normal Text
With lists and paragramps, are not altered
Within triple Quotes
"""
I expect the main thing you are missing is the shere power available in TW5 and its use of related technology. Cross Platform (Mobile and Tablet) and Server backends are only the beginning.
Regards
Tony