What has been your biggest challenge in implementing GTD?

I have two challenges:
1- Actually getting things done. At the end of each day I always have tasks that need to be transferred to the next day, and I find that frustrating. I don't know if I need to schedule fewer tasks or just not worry about it. Advice?
2 - Doing the weekly review. That 1 1/2 hour commitment is very hard for me to carve out. I realize it's essential, but I find I slack on that too often.

I use Omnifocus used to put target dates for the "important" things because I wanted them to stay top-of-the-list and not get mixed in with 100 other things. Then like you I would find myself modifying that date on "overdue" actions to move them into the future. They weren't really overdue...I just hadn't made the arbitrary date I had assigned. I've changed this by taking advantage of the "flag" functionality, which lets me mark something as important without setting an arbitrary due date. It saves me the time and deflation of going back to adjust due dates. And I can still scan all of my flagged items when it is time to reflect & engage.

This is a great idea to use flags. ToDoIst allows me to attach priorities to posts. Maybe if I put the URGENT tasks as Priority 1 and unprioritize the rest it will be easier on my psyche. There may be other tools in the app that I could consider also. Thx @LeeAase

It's been hard for me to remember to record items as soon as I think of them. I find myself "dumping" a lot of items at once. :/

I agree, Audrey Laine! I find it hard to always remember to record either to-do or waiting for items. Recently, I've told myself to slow down on replying to emails and that helps me to be more mindful if I have additional action items to note from replying to a particular email.

My biggest challenge is having enough time to put all the "to-dos" down and to sort through my inbox (which is the top of my desk). Ugh.
I have made good progress about keeping my to do list active in Nirvana, but its not 100% yet.

I needed a system that was completely mobile. It took a while for me to find the system that worked for me but 'Todoist' was the winner. Now my biggest challenge is remembering to think through the 'context' portion when I am clarifying. Context = where do I need to be to do the item on my action list so I group my to-do's by location instead of random jumping from item to item.

I'm a big ToDoist fan too! Highly recommend it. What do you like best about it, @diananordrum?

There are two things that I like best - the inbox & the filters. The inbox is great when I just need to do the mind sweep. The filters are great because I can customize the filter to find just those items that I want to view at a specific time. How about you?

Those are both great features, @diananordrum. I really like how easy it is to add a task on mobile when I'm out and about and something pops into my mind. I can even ask Siri to add it. Also forwarding emails to the program. Adding notes. Assigning tasks to "tomorrow" or "next week" or "Friday" is so simple. I'm a believer.