1. A wide range of archetypes/classes and associated powers/capability combinations to choose from, with plenty of 'alt' space in the account
*LOTRO has 10 classes (and 4 races), each of which have various "specializations" you can use to focus (a damage heavy caster vs a pet caster, for example).
*LOTRO comes with 5 character slots (Shadows of Angmar version) per server, with 2 more available (per server) with Mines of Moria (currently -the- available box), and 2 more (per server) upcoming with the Adventurer's Pack.
*DDO has 11 classes (2 of which you have to pay for), and 7 races (2 of which are purchasable) with the option to multiclass (fighter/rogue, bard/sorcerer).
*DDO you get 2 slots to start per server with the option to buy more. If you subscribe you get 4 slots with the option to buy more (up to 10 total)
2. Full early/mid game content as well as open-ended end game content (more the journey than the destination, or even no destination)
*LOTRO has 4 "starting" areas, depending on race, unique "flavors" of quest for each race (hobbits tend to have kinder, gentler quests, while dwarves have burly goblin-smashing quests). You cannot outlevel quests in LOTRO (however XP is negligible if the quest is grey) and there are multiple deeds to achieve in each area. There are raids for those who like that, but they're not necessary to succeed in the game. There are also daily quests, bounty quests, and raid quests that keep high level characters playable with purpose.
*DDO starts everyone in the same area, with the same quests. After you progress into the city proper, there are multiple levels of stuff to do (with purchasable quest packs available).
3. A logical, balanced and robust game mechanics and combat resolution system, with a reasonably intelligent (possibly player-adjusted for difficulty) AI.
*LOTRO is getting an overhaul of its combat system with Siege of Mirkwood, and it makes combat very fast, very dynamic, and pretty interesting. The AI is fairly sharp, if predictable.
*DDO is real time combat. Your enemy fires an arrow - react quick enough to duck behind a box, and the arrow won't hit you. Blocking and tumbling have an effect on combat. DDOs AI is also sharp, given that it's the same company.
4. Ease of solo ability as well as online team play.
*LOTRO is very soloable with certain classes, and difficult soloability with other classes. When creating a character there's a nice little "Solo ability" label with each. Grouping is easy, but not required, and there is a built in voice for those that use it (personally, I don't).
*DDO is much less soloable, but it's still possible. Some objectives may require certain classes due to the nature of the game (quests with multiple, deadly traps needing someone with a disarm traps skill, for example). Multiclassing and other options are available to get around or just bullrush through some things.
5. Varied, multi-branched quests, storylines, campaigns/arcs, with choices even at the lower levels.
*LOTRO, again, has 4 starting zones and no race is required to stay in their starting zone. You can make an elf and immediately travel to the Shire to do the hobbit lowbie quests. Although the storyline roughly parallels the journey of the Ring, it's not a straight and narrow. For the Tolkien fan, LOTRO brings the Middle-Earth to life, not just the areas dealing with the path of the ring.
*DDO is pretty much restricted to Stormreach. While there are areas outside of it, they tend to be for higher level/group/raid types. Also, many are now restricted from those who have not purchased that particular pack.
6. An integrated economy and crafting/building capability, but not so dominant that it detracts too much from play time or becomes a requirement to create an effective character at their relative level
*LOTRO has multiple crafting paths, from weaponsmithing to cooking to woodworking and more, with items that make sense to use (ash wood to make an ash bow). Items made by crafting CAN be better than items received as random drops from mobs, but as the system itself is easy and fun, just about anyone knows someone who is a Grand Master XXXX who will make something for them. The market is generally used by those who are looking for quest items, raw materials or recipes, rather than finished product, although it is possible to find high-grade items for reasonable prices. It also doesn't take long to figure out when something is over priced. Auctions have a set period of time, chosen by the person placing the item up.
*DDO doesn't have crafting the way LOTRO does, however there is a sort of system involving these shards that you can take to a certain place in Stormreach, combine them, and hope you get something lucky, even though it takes thousands of shards to really get anything useful. There is also an auction house which is mostly haphazard and useless.
7. Difficulty level adjustable by player (from easy to impossible, least to most, with more than one variable)
*LOTRO does not have "difficulty" levels. The closest it comes to such is that the quests are rated in the universal con colors (purple for the ones way over your level, and gray for those way under). If you really want a challenge, taking red or orange quests will provide. Quests will also be rated as solo, small fellowship (3 people) or full fellowship (6 people).
*DDO does allow you to run through a quest multiple times at increasing difficulties, depending on your team size. If you are a solo player, you'd need to first run a quest at solo or normal difficulty, then again at hard, then again at elite to do them all. There are experience bonuses for completing a mission the first time on a certain level which helps to aid the repetitiveness of it. Larger teams will automatically have access to the hard and elite options.
8. Design-your-own (DYO) quests/battles/missions/maps/enemies
*LOTRO, with the release of Siege of Mirkwood, will introduce the ability to create skirmishes to play. You can play them any time, from anywhere, via a slash command interface. You can choose the type and kind of skirmish, but that's about all. You cannot make missions for other people to play. Think of it as a newspaper/radio mission generator with control over where and what you fight.
*DDO does not have any kind of quest customization.
9. Preference to buy game, then play free or buy/download game, then play a flat subscription fee without having constantly pay for extras to improve/enhance characters (i.e., how good your characters can be/look does not depend on paying RL money for extras).
*LOTRO has multiple pricing options including an occasional lifetime subscription option that runs at 199 which, in my opinion, is more than worth it. Prices are typical of the average MMO, from 10 - 15 dollars a month depending on your deal. The current bundle includes Mines of Moria (the first expansion). Siege of Mirkwood is a digital expansion and will be out in December. The Adventurer's Pack is a bundle of goodies including a mount, 2 character slots, and shared bank slots. There are frequent book updates, which equate to CoH's free issues.
*DDO is free, however you would need to pay if you wanted to play certain races, certain areas, or get certain items - none of which are REQUIRED to play and enjoy the game. The stuff that is bought is not so incredibly uber it makes game drops worthless, nor the free play quests less fun. Alternatively, you can get VIP access to DDO for 14.95 a month which will give you access to everything the free player would have to purchase. In addition, you earn coin that can be spent in the DDO store to purchase things (quest packs, items, races, classes) instead of using cash/subscribing. VIPs get a certain amount per month, free players can earn coin by earning faction or purchasing coin bundles.
I hope this sufficiently answers the questions, at least in regards to these two MMOs *whew*.