Heretic and Hexen came along, and I was playing them a lot as well. They were very cool for a D&D fan such as myself, but others who weren't into the fantasy genre dismissed them as "Doom in tights".
A couple of features I picked up on which I have sometimes included in my games of D&D:
- Healing potions can be common treasure, and will keep PCs going long after they've suffered their total hp in damage. This may well have a knock-on effect on tactics and strategy once PCs realise this.
- Tactical situations can be more interesting and difficult than variety of monsters.
- Monsters can fall foul of traps as well, and each other if their intelligence isn't too good.
- Environmental hazards, even when obvious, can still be awkward to get around or painful to get through
- FPS and other 3-D games can use quite complex room lay-outs that may be difficult to adjudicate in a pencil-and-paper game unless you have a 3-D battlemap. Nonetheless, some of the interesting rooms and other areas can be used in D&D.
The next big thing to come along was Diablo. I still sometimes play Diablo II, particularly when my internet is down. The stuff I could really appreciate was the wide range of new monsters - very cool. The idea of equipment being useable depending on the character's level and ability scores is sometimes tempting, but I don't think D&D players would appreciate finding powerful items that they couldn't use yet.
Scrolls of identifcation (to use on magic items) and town portal scrolls (creates a temporary magical portal that enables a return trip to your home base and back to the dungeon) are again interesting ideas that might speed up play, but I'm not sure whether they would be abused, or become common enough that players kick up a fuss when they're not available.
It's interesting that I first noticed combinatorial magic items in Diablo, and then they were introduced into 3rd Edition D&D. Rather than just having another battleaxe +1, a DM could give it properties such as cleaving or holy that could be applied to any weapon.
Finally, my current favourite - World of Warcraft. World of Warcraft has certainly been influenced by D&D - they borrowed gnolls and kobolds. In return, I'm sure lots of D&D players have borrowed monsters and characters from WOW, including the Draenai and Blood Elves. The things that strike me about World of Warcraft, as a D&D player/DM include:
- The quest system is a lot looser than in Diablo, and you can pick and choose which quests you do. It's basically a sand-box setting. Although some of the quests are so simple as to be boring, others are interesting enough to inspire D&D adventures.
- The professions/skills system is a lot more complicated and in-depth than in any edition of D&D. This is because each skill needs to scale up in usefulness as a player progresses to very high levels otherwise it ceases to be of interest to the players. Applying that sort of skill/profession advancement in D&D, but also realize that the WOW encumbrance system is not very realistic - if you skinned a large monster such as a wyvern or a hydra, do you really think you could simply fit the hide into your backpack? A stack of 20 of them?
- The one drawback of WOW that D&D does not have is that players cannot permanently affect the world around them - only themselves. Sure, they can kill whatever bad guys they want, but respawning means that the bad guys will always be back. Similarly there are no permanent consequences for PC failures at quests (except maybe to their own advancement), so a troublesome quest can be abandoned without caring about the effect on NPCs.
- Chatting and cooperating with other PCs online is great fun, particularly when in a party, But it's a pity that NPCs aren't very interactive. I suppose that would be asking too much. It's just that in D&D, NPCs are controlled by the DM, so they can be as flexible and interactive as the PCs.
- The idea of different areas being geared towards different ranges of levels is something that could work in a D&D campaign, but it might become a bit obvious, and spoil the suspension of disbelief. Mind you, a lot of the areas are actually very nicely done, particularly in the expansion packs. Weird places like the Blades Edge Mountains need to be seen to be believed but setting D&D adventures in similar places would be cool.
- Monsters (including hostile humanoids and animals) scaling up in power through the levels needs a bit of discretion. Just out of convenience, I use the rule of thumb that 1 D&D level is approximately 3 WOW levels. So fighting level 8 bears in Elwyn Forest is like taking on a 2 or 3 HD bear in D&D. In the Grizzly Hills in Northrend, the bears are level 75, like having a bear in D&D with 25 HD. With humanoids having levels in character classes, this is a bit more interesting and a bit more believable (for me at least). Send your PCs up against the orcs of Hellfire Peninsula and see how they cope with orcish 20th level fighters and 20th level priests. Of course, they may wonder why the orcs are so high-powered but not leading huge tribes of their own - a valid question which the DM may not have an answer for.
I think the conclusion I have often reached is by all means borrow from computer games - they've got some great ideas. But also be aware that not all ideas will fit into D&D comfortably.