A summary of differences between D1.5:2008 and D1.1:2008 (feel free to add to this list). In short D1.1 and D1.5 are different enough that when you switch between the codes you need to leave any preconceived notions behind otherwise you will likely get burned.

WELDING PROCESSES
In D1.5, only SMAW WPS's are pre-qualified, as long as they are in accordance with other sections of the code.  FCAW and SAW WPS's are not pre-qualified and have to be tested. D1.5 doesn't provide for prequalified welding except with Low hydrogen SMAW electrodes. In fact the commentary section does a nice job explaining it clearly.  "C-1.3.1 A major difference between this code and AWS D1.1 for welding primary members and connections is that only WPS's using listed SMAW electrodes with a specified minimum yield strngth less than [90 ksi] are to be considered pre-qualified and exempt from testing in this code.  Other WPS's for primary connections are to be qualified as described per clause 5."
D1.1 allows a broarder range of pre-qualifed WPS's.

FABRICATOR CERTIFICATION
D1.5 requires bridge fabricators to be AISC certified or equilevent.
D1.1 doesn't have this requirment.

CALIBRATION OF EQUIPMENT
D1.5 requires that, "The contractor shall verify, at least every 3 months, the accuracy of meters and other devices used to record or display welding variables."
D1.1 only requires that welding equipment be kept in such condition as to enable personnel to follow procedures and obtain required results. No specific calibration interval is mandated.

WPS REQUALIFICATION DUE TO TIME LIMITS
D1.5 requires that all WPS's, except those specifically exempt, shall be based on tests which have been performed not more than 60 months in advance of production welding.
In D1.5 for FCM, PQR's are good for only 36 months.
D1.1 does not have a limitation on the time frame they are good for.

CHIPPING AND BRUSHING DURING WELDER TESTING
In D1.5, the welder can't grind on his test plate during the test.The no grinding requirement on welder qualifications is often misunderstood.  That does not mean that grinding is prohibited on production work, just on the test (although major grinding could be weld removal and that requires engineer's approval). Excessive grinding and re-welding in production could adversely affect the steel.
D1.1 does not matter if you grind on the test plate.

WITNESS FOR WPS QUALIFICATION
D1.5 requires that "Welding and machining shall be witnessed by a state representative or an independant third party acceptable to the state."
D1.1 does not require a state nor third party to witness welding of a WPS plate.

FILLET WELDS - MULITPLE PASS LIMITATIONS
IN D1.5, up to 5/16" fillets must be single pass welds (heat input control).
D1.1, does not have a lower limit on multiple pass weld size.

WPS QUALIFICATION
In D1.5 you can either qualify WPSs by testing by controlling heat input requirements (5.12) or by essential variables (5.13). You could also qualify a D1.5 maximum heat input PQR, or a min/max heat input PQR. D1.5-5.13 allows amps at +/-10%, volts +/- 7%, travel +/- 10%, and heat input is +10%/-30%. (These are for FCAW and there are other differences I didn't mention.) The point is that heat input control is important with D1.5.
In D1.1 you control WPSs through essential variables. With D1.1 PQR amps can vary by +/- 10%, volts +/- 7% , travel +/- 25%, and heat input by + 10% (no limit shown for reduction in heat input). A lot of D1.1 welding is prequalified if within the manufacturer's recommended ranges; which are pretty wide ranges.

BASE METALS
D1.5 wants you to use ASTM A709 (AASHTO M270) steels. It makes is very difficult to use steels other than A709 by making the qualification of those unlisted steels more difficult than qualifying unlisted steels in D1.1.
D1.1 requires the qualification of unlisted steels, just as D1.5 does, but it does not require evidence of the steels acceptability to other codes, a minimum history of five years use under similar conditions of loading, past records showing the steels resistance to hydrogen cracking, and history of max/min heat inputs for each process to be used.

OPTIONAL HORIZONTAL PLATE - WELDER QUALIFICATION - UNLIMITED THICKNESS
D1.5 requires the plate to be a minimum of 5-1/4 in.
D1.1 requires the plate to be a minimum of 5 in.

PRETESTING WPSs
D1.5 allows as contractor to use a PQR qualified by someone else as the basis of a WPS if he performs a verification test.
D1.1 requires to the contractor to run all PQRs himself.

VERIFICATION TESTING
D1.5 allows for a simplified qualification test (verification test) if the contractor wants to use a PQR obtained from a third party as the basis for a WPS.
D1.1 does not allow for verification testing.

WPS QUALIFICATION TEST PLATES
Between D1.5 and D1.1, the minimum lengths of the plates and location of the specimens are different enough to make an item by item comparison cumbersome in this particular list. In short they are similar but they are not the same.

PREQUALIFIED JOINT DETAILS - SINGLE BEVEL AND DOUBLE BEVEL JOINTS
The D1.5, in many cases, is much more restrictive on the allowable positions for single and bevel joint. For instance, B-U4a can only be used in F and H positions.
D1.1 is more permissive with the allowable positions for single and bevel joint. For instance, B-U4a can only be used in ALL positions.

...that's all I have time for now. Again feel free to copy the above and add to it. When the thread gets too long someone should recopy to the top of a new thread.