To preface this, I played DD mostly with my wife Liz, and then later a little with more hardcore friends.
DD was the first game Liz REALLY liked to play, to the point where if she had free time, she'd actively seek me out to play. This was during the beginning of the games life and we were working our way through the campaign. Her play style is that she likes to have an area she builds towers in, does repairs, and defends a crystal. She has NO ABILITY to navigate in a 3D environment when under duress, getting lost in pretty much every level ever so we had to kind of split up levels of where she could stay, and I had to do the majority of the running around. She got better as we went, handling 2 crystals that were in line of sight to each other or a direct path.
Here are the things that really stopped us from playing more of DD though.
1. Timers on building. We'd beaten all of Hard difficulty, but once we did insane and it put in the build timers, it immediately killed any further progress for us since Liz wanted my opinion on tower placement fairly often, and we simply couldn't cover some of the larger maps chest quickly enough to stop the first or second wave also due to navigation issues.
I'm not saying timers are bad or should be completely removed, but they should be an optional setting similar to survival and hardcore.
2. Maps that took more then 45 minutes to complete. We're both adults, we both have responsibilities after work and if we're lucky we squeeze in an hour or so at best each night. The only way to play harder difficulties with no timer was Survival mode. But as people have said, this ended up being 2-3 hour investments with little player input for marginally better gear. This wasn't fun for her and what led to us eventually moving on past DD as a game we both really liked, but obviously the late game wasn't for our play style.
3. 4+ crystal maps with 2 players. These we mostly just ended up skipping as they were impossible with just 2 players so we could never really complete all the maps. I know people say play with others, but most people don't like playing with the slow players. I would of liked to see some harder 2 crystal maps, but almost all the new content was 4+ crystals.
Later on I played with some friends who were much more hard core about the game, and single so they had more time to grind. Here's some of the issues we ran into that stopped us from progressing much after a while.
1. Level/Equipment disparities. This is what hampered most players later, some one in your group spent an entire weekend grinding, was 10 levels higher with gear 20x better and so anything that was a challenge to them was instant death to everyone else, anything challenging to the rest, they could solo while sleeping.
If there was some way to balance the stats across players so the game could remain a challenge for all that would be great. One idea if to have the option to simply average the stats of all players, so everyone simply gets the stats they invested in boosted to an equal level, and those who are over powered have their high stats spread out among others and they themselves are lowered back down to the groups. Again, option for casual groups, not a default settings for pubs.
2. Some classes became redundant. My hard core group pretty much had a high level tower mage and squire, we didn't need anyone else building towers. I eventually switched to a barbarian for this reason, and it was okay but he never felt like he contributed much since he was behind in levels since I had to start him much much later. We tried out the summoner a bit too, but while he didn't use DU's the cost of mana taking away from tower upgrading was an issue at the much much higher difficulty levels and again, he was too far behind to catch up due to the level grind at the end being very very rough (I have all the classes, we only ever got 1 to 78, and 3 others to 74-76, the rest might have gotten to 70 or less).
Trendy we are both looking forward to DD2 and hope you can take our issues to heart, please think of us when considering difficulty jumps and a few options to customize games for smaller groups. Also, please PLEASE keep split-screen co-op as an option, as thats vital to any game we play together.
Jonathan and Liz