Computer RPG's have:
- Limitation of Setting
- Limitation of action
- Limitation of mechanics
- Limitation of dialogue
- inability to restrict to sensible interactions
- Ability to Save = Lack of Risk
- Hardware and Power
- Illusion of Choice
In a CRPG, the setting is fixed by the programmer. Occasionally (NWN, Realmz), it can be changed by conversion packs, but the fundamental axioms are still often hard coded. In a TTRPG, the setting can be changed by the GM fairly easily - it's one of the more common things to do, in fact.
In a CRPG, your action choices are limited to those selected by the programmer and story team as possible. In many newer games, this can be a quite extensive lost, but it's still pick from a list. In a TTRPG, the rules may only cover a specific list, but most provide for the GM to adjudicate anything the player decides to have his character attempt. (A few do not - usually for genre reasons - eg: Golden Sky forbids physical attacks as a blanket prohibition, as they're out of genre.)
To be blunt, however, this is very often not much more open in actual TT play - many GM's restrict actions, especially in combat, to a limited list. This actually boils down to interface - in CRPG, the interface usually tells you what you can attempt, while in TTRPG, even when your choices are limited, you are not generally confronted with a list. You can often describe an action, and the GM will convert it to whatever the closest "on the list" action is.
In a CRPG, the mechanics are limited solely to those included in the codebase, intended or not. (Yes, bugs sometimes allow actions other than those the design and programming team want.) If one of the mechanics annoys you, you're stuck with it. If one has bad simulation, you're stuck with it. If it's lacking an important mechanic, you're stuck without it.
In a TTRPG, the GM (or the group) can modify the rules as needed. Often, the GM modifies the mechanics unintentionally - misunderstanding of rules, misremembering of rules, and assumption that it's the same as in another game often lead to unique flavors for every GM.
In a CRPG, dialogue is the most limiting aspect. You usually cannot say anything that wasn't pre-programmed; in a few, you can't say anything at all (like the Zelda games). In a very rare few, you can say what you want, but only certain key phrases get responses.
In a TTRPG, you can say whatever you want. The GM usually can parse it, and react accordingly. (A few rare exceptions exist - eg: Parsely restricts the GM to a keyword list.)
In a CRPG, sometimes the allowed options include some non-sensical ones for certain situations. Sometimes this is lazy coding, sometimes it's due to reskinning an extant engine, and sometimes, it's an intentional illusion of freedom, and rarely, it's an intentional effect to simulate intoxication or insanity; most often, tho', it's a bug.
In TTRPG, non-sensical interaction choices are almost never presented explicitly, so when they are, it's almost always an intentional puzzle to be figured out. (Examples include a dungeon set up as a hypercube, alien dimensions, and prophetic dreams.)
Sense of Risk
In a CRPG, most of the time, one has access to a save feature. It varies where and when, but usually, it's coded such that one can restore from an old save. The sense of Risk is thus very illusory, if present at all. Even in the few coded against restorign from old saves, one can usually back up the save file. The primary exception being on cartridge games, such as those for Nintendo's hand-helds.
Hardware and Power
CRPG's require a computer. And a computer requires electrical power. No power, no play. Even on a handheld, when the batteries are drained, you're done.
TTRPGs require only that you have players and a GM, and enough light to read and write as needed, which can be as low as a single candle. In some cases, TTRPG's can even be played in total darkness - feeling the pips on d6's, and keeping character abilities either with tangible markers or in memory.
TTRPGs can be played equally well by the blind, the deaf, and those lacking dexterity - the only requirement is to be able to communicate, and that at least one person in the group be able to make annotations as needed.
The Illusion of Choice
Fundamentally, playing a predesigned adventure to conclusion is much the same in either. You have a series of required encounters to pass, and a series of actions to take to accomplish those, and often, freedom to work within encounters by a couple different approaches to the encounter.
In CRPGs, the appearance of choice is much more restricted, simply by the interface. If you don't see it, you usually can't try it (but there are exceptions to this).
In TTRPGs, the appearance of choice is much more open, even when the GM is restricting to stuff covered by the rules, because the GM doesn't always need to say, "that's really this rule" - the GM just uses the rule to resolve it, and reports the result. And in some people's games, the freedom of choice isn't even illusory; you can have your character try anything, and the GM will apply common sense and rules to adjudicate it, hopefully in a fair manner.
Fundamentally, most CRPGs are single-player experiences. Even in MMO's, the typical experience is that of being alone at your computer, possibly interacting with others, but still being limited by the system. Even when a group gathers together for a LAN party while playing, the experience is still focused upon separate screens.
The fundamental experience of a TTRPG played in a traditional mode — 3-20 people in one room — is one of a group activity. Everyone interacts with the GM. Everyone can interact with each other, as well, both in and out of character.