Here's the Defensive side of my Importance of Positions post. Note: This is based on a 4-3 defence.

1. Middle Linebacker

This was a tough call but in the end, the MLB wins because it acts as the QB of the Defence. They provide the leadership and all-round qualities that are vital on the defensive side of the ball. A good MLB can do everything- they can blitz the QB, cover the middle of the field and, to some most importantly, stop the run. The RB's worst enemy is the MLB as they protect the middle of the field and also have the range to pull across to the sidelines.

A weak MLB doesn't have the effect of a weak QB however. Other positions can make up for the lack of a MLB. DE's provide the rush, DT's stop the run and DB's cover the pass plus the OLB's can do everything a MLB can. While this gap can be covered, it still does leave a hole in your defence which needs to plugged at some time. Another knock on the MLB position is that few MLB's can do all the things I've outlined. Most are just dominent rushers and lack coverage skills. In the end though, a MLB can provide the everything your defence needs.
Do the Lions have a good MLB?- No.

2. Defensive Ends

The Defensive End's importance comes from the fact that he is the biggest problem for a QB, the most important offensive player. A good DE can get to the QB cutting his decision-making time down as well as providing sacks which brings offensive drives to a halt. Good DE's can also make stops on the RB, preventing rushes on their egde.

A weak DE provides the exact opposite of a strong one. No pressure leads to lots of time for a QB to make a decision and good lanes for a RB. This puts significant pressure on the secondary leading to poor perfomance by the whole defence.
Do the Lions have good DE's?- For the most part, White has been a solid veteren and Avril appears to have lots of potential.

3. Cornerbacks

Corner's have a vital job on the defence but are somewhat isolated. Corners mostly have one on one battles with their opposing WR. A good CB does bring a lot to a team though. As someone like Deon Sanders proved, being a dominent corner can completely take a receiver out of the game and prevent a QB from throwing in your area of the field. If a corner can provide such an advantage, the defence goes a long way to being successful. A good Corner can also force turnovers via interceptions which, if athletic enough, can turn into TDs.

A weak CB can lead to easy TDs for the offense, making decent WR's get open easier and giving QB's simple options to throw to. Having weak CB's on your team is a major disadvantage and you won't get to far without them, unless you have brilliance in other areas.
Do the Lions have good CB's?- Not right now. Buchanon was a good FA pick-up but Henry is old and better off as a Safety.

4. Defensive Tackles

Defensive Tackles provide the biggest presence on the field for the defence. Their size is used to plug running lanes and get as much penetration into the backfield as possible. A good DT can completely disrupt an Offensive Line and collapse the pocket keeping the QB on the move  as well as limiting the Running backs gaps, preventing large runs. But a DT can do alot more for others, a strong DT can require a double team of blocker's to subdue leaving LB's holes to blitz through.

On the flip side, a weak DT will not be able to prevent a dominent push from the Offensive line on run plays and will provide zero support on blitzs. The biggest problem however is that poor DT's negatively affect LB effectiveness and DE effectiveness. A team without good DT's is a team with a big hole, but the hole can be covered with good coverage and good schemes.
Do the Lions have good DT's?- Not really, Grady Jackson is a stop gap and Darby isn't very good at all.

5. Outside Linebackers

Outside Linebacker's are something of a mixture between DEs and CBs. Most OLB's are used as specialists. Some are pass rusher's who are virtually undersized DE's while other's are good at coverage and are like big Corners. Their importance comes from being able to be used in a multitude of situations and in an all-round compacity like a MLB. However, some OLB's are not on the field all the time with many being taken of on pass plays for DB's. Good OLB's though can provide lots of sacks, lots of tackles and a couple of INTs.

Weak OLB's don't provide as much of a problem as poor MLB's do. Like I said before, many of them are specialist's who provide an extra punch. They can't rush like a DE or cover like a corner. In the end, a weak OLB has the least effect on the defence since they can be substituted out through diffferent formations.
Do the Lions have good OLB's- Yes. Sims is still young and still has upside while Peterson, a new FA pick-up, has great pass rushing capabilities and good coeverage skills.

6. Safeties

Safeties are a position that have no real specific role. They are mainly used to support the CB's and provide extra help in the run but do not specific cover any one position nor do they specifically cover the run. They are 'Safeties', help for a certain part of the Defence. A good Safety does bring a lot though. Most are actually playmakers for the defence with some being slight guys who have good hands and can make picks while some are bigger lads that prefer to intimidate with big hits. Offence's take notice of a good Safety and are always on the lookout for them since they play all over the field.

The reason I consider this the least important position on defence is because Safeties provide nothing crucial for the defence. A weak Safety doesn't require another position to step up it's game, it just means that the Defence won't get any extra help in coverage or the run. It doesn't mean you won't feel the lack of a Safety, it just means you'll feel it less.
Do the Lions have good S's- Debatable. Bullocks has showed ability at times and Alexander hasn't had decent time on the field. We shall see.

Some of you maybe asking, "What about the Kicker or the Punter?". Well, they are important positions but because they play such little amounts of time on the field, their importance is dwindled. A good K or a good P should not be underestimated.